Evaluation of Environmental, Nutritional, and Host Factors in Cats with Hyperthyroidism

Morbidity of insured Swedish cats between 2011 and 2016: Comparing disease risk in domestic crosses and purebreds

BACKGROUND

Sources of population-based cat health information are scarce. The objective of this study was to determine disease frequency in cats using pet insurance data to inform health promotion efforts.

METHODS

A descriptive analysis of cats insured with Agria Pet Insurance in Sweden (2011-2016) was performed. Incidence rates of broad disease categories were calculated based on veterinary care events and an exact denominator consisting of cat-years-at-risk. Rate ratios were calculated, comparing domestic crosses to all purebreds and specific purebreds to all other purebreds combined.

RESULTS

The study included over 1.6 million cat-years-at-risk (78.5% were domestic crosses), 18 breeds and 24 disease categories. The most common disease categories causing morbidity in purebreds were digestive, whole body, injury, urinary lower, skin and female reproduction. Purebreds had the highest relative risk (compared to domestics crosses) in the female reproduction, heart, operation complication, respiratory lower and immunological disease categories.

LIMITATIONS

There are typical limitations of secondary data, but they do not negate the overall value of such a large dataset.

CONCLUSION

This study demonstrates how pet insurance data can be used to find breed-specific differences in the incidence of various disease categories in cats. This may be of importance for breeders, cat owners, veterinarians and researchers.

Estimating short and longer-term exposure of domestic cats to dietary iodine fluctuation

Hyperthyroidism is a common endocrinopathy of domestic felines. In humans, toxic nodular goitre is pathophysiologically similar to feline hyperthyroidism and can be caused by chronically low or fluctuating dietary iodine intake. The aetiopathogenesis of feline hyperthyroidism is not known, but chronically low or fluctuating dietary iodine intake is likely common. This study assessed habitual iodine intake in domestic cats by: (1) conducting a dietary survey involving 361 owners of 549 cats, (2) analysing iodine content of 119 cat feeds, 38 urine and 64 hair samples and (3) assessing variation in iodine content of eight cat feeds over 4–6 different batches. Owners varied their cats feed regularly, usually on a day-to-day basis and often between wet and dry feeds with differing flavours. The majority (78%; 93 of 119) feeds for cats were within the guideline range for iodine. Of the 22% (n = 26 feeds) that were not compliant, the majority (n = 23) were below the nutritional minimum value with most (n = 16) being dry kibble. Iodine content of feeds did not vary considerably between types of feed or feed packaging, but variation between different batches of the same feed was 14–31%. Hence, urine iodine in cats also varied markedly. Cats being treated for hyperthyroidism had lower hair iodine. In conclusion, a survey assessing how domestic cats are fed, together with an analysis of commercial cat feeds suggests that domestic cats are likely to experience chronically low or fluctuating dietary iodine intake. The latter is supported by wide variation in urine iodine content.

Hyperthyroidism in the Domestic Cat (Felis catus): Informed Treatment Choice Based on Survival Analysis

Hyperthyroidism is the most frequently diagnosed endocrine disorder in cats. Therapy may include pharmacological, surgical (thyroidectomy), radioactive (iodine), and dietary treatment. The choice of treatment is believed to be strongly influenced by the veterinarian’s experience, level of education, and knowledge of the current scientific literature. The history of survival rates can affect the decision for treatment by both the veterinarian and the owner. This study aimed to explore the longevity in cats diagnosed with hyperthyroidism and to identify significant variables which affect survival rates by using retrospective data from the practice. A multivariate Cox regression was applied with the following results: surgical thyroidectomy and methimazole medication produced similar longevity (median 23.5 months, P>0.05); Domestic Short Hair cats survived longer than pure breeds (median 27.2 vs 9.4 months, P<0.05); as do cats without chronic renal disease (median 28.1 vs 6.2 months, P<0.001); and those with low activities of alanine aminotransferase (median 27.1 vs 17.0 months, P<0.01). Hyperthyroidism is comorbid with renal diseases, but no cumulative effect was found on survivability. There was no difference in survival rates between surgical and pharmaceutical treatment, therefore the discussion of treatment options with owners can focus on other factors (e.g., cost, owners’ compliance, cats’ tolerance to medication, presence of comorbidities). We propose that surgery may be the preferred treatment when the survival rates are expected to be higher than one year. This would avoid high costs and potential side effects of medication.

Determination of bisphenol A in commercial cat food marketed in the Czech Republic

OBJECTIVES

Bisphenol A (BPA) is one of the most widely used synthetic compounds on the planet. It is used in the synthesis of polycarbonate plastics, epoxy resins and other polymer materials. Owing to its excellent chemical and physical properties, it is used to produce food and beverage containers or the linings for metal products. BPA has been mentioned as a possible cause of feline hyperthyroidism. Cat food is considered one of the main sources of BPA intake. The purpose of this study was to evaluate BPA concentration in various types of commercial cat food available in the Czech Republic.

METHODS

In total, 172 samples prepared from 86 different types of commercial cat food were assessed. The concentration of BPA was measured using liquid chromatography-tandem mass spectrometry.

RESULTS

Measurable concentration of BPA was found in all samples (range 0.065-131 ng/g), with the highest concentration (mean ± SD) of BPA in canned food (24.6 ± 34.8 ng/g). When comparing BPA concentration in food trays (1.58 ± 0.974 ng/g), pouches (0.591 ± 0.592 ng/g) and dry food (1.18 ± 0.518 ng/g), concentrations of BPA in food trays and dry food were significantly higher (P <0.01) compared with pouches. Comparing BPA concentrations in canned food of different manufacturers, statistically significant differences were found as well.

CONCLUSIONS AND RELEVANCE

The highest concentrations of BPA were found in cans. Thus, cans represent the highest possibility of exposure to BPA in comparison with other types of commercial feline food.

Serum concentration of bisphenol A in elderly cats and its association with clinicopathological findings

OBJECTIVES

Bisphenol A (BPA) has been mentioned as a possible factor contributing to feline hyperthyroidism. Nevertheless, there are no previous studies reporting on the concentration of BPA in feline serum and its association with thyroid function. The objectives of this study were to measure serum BPA concentration in cats aged ⩾7 years, considered as healthy by their owners, and to compare the results with clinicopathological findings.

METHODS

Sixty-nine cats aged ⩾7 years considered as healthy by their owners were enrolled in the study. The concentration of BPA in feline serum was measured using liquid chromatography-tandem mass spectrometry. In all cats, signalment, living environment, diet history, and the results of haematological and biochemical analysis, including thyroxine levels, were available.

RESULTS

The mean serum BPA concentration in feline serum was 1.06 ± 0.908 ng/ml. Significant correlation was found between BPA concentration and haemoglobin (r = 0.3397; P = 0.0043), haematocrit (r = 0.3245; P = 0.0065) and the number of red blood cells (r = 0.2916; P = 0.0151), concentration of total protein (r = 0.2383; P = 0.0486), concentration of calcium (r = 0.3915; P = 0.0009) and level of bilirubin (r = 0.3848; P = 0.0011). No other significant correlations were found. Significant differences (P <0.01) were found between mature (1.28 ± 0.994 ng/ml) and geriatric cats (0.420 ± 0.240 ng/ml), between strictly indoor cats (1.27 ± 0.992 ng/ml) and cats with outdoor access (0.660 ± 0.529 ng/ml), and between cats fed canned food (1.23 ± 0.935 ng/ml) and cats fed non-canned food (0.774 ± 0.795 ng/ml).

CONCLUSIONS AND RELEVANCE

Measurable serum BPA levels were found in all examined samples. The age of the cats was revealed as a significant factor affecting BPA concentration and mature cats had the highest levels. A significantly higher concentration of BPA was found in cats living strictly indoors and in cats fed canned food. No association was found between BPA and thyroid function. Further studies are needed that focus on hyperthyroid cats for better evaluation of this relationship.

Are persistent organic pollutants important in the etiology of feline hyperthyroidism? A review

Feline hyperthyroidism is a rather new disease, first reported from the North American east coast in 1979. The prevalence is increasing, especially in older cats, and hyperthyroidism is now reported worldwide as the most common feline endocrinopathy. Several studies have been performed trying to identify important etiological factors such as exposure to persistent organic pollutants, and especially brominated flame retardants, have been suggested to be of importance for the development of the disease. Recent studies have shown higher concentrations of these contaminants in serum of hyperthyroid cats in comparison to cats with normal thyroid status. However, other still unknown factors are most probably of importance for the development of this disease.

Silicone Pet Tags Associate Tris(1,3-dichloro-2-isopropyl) Phosphate Exposures with Feline Hyperthyroidism

Feline hyperthyroidism is the most commonly diagnosed endocrine-related disease among senior and geriatric housecats, but the causes remain unknown. Exposure to endocrine-disrupting compounds with thyroid targets, such as flame retardants (FRs), may contribute to disease development. Silicone passive sampling devices, or pet tags, quantitatively assessed the bioavailable FR exposures of 78 cats (≥7 y) in New York and Oregon using gas chromatography-mass spectrometry. Pet tags were analyzed for 36 polybrominated diphenyl ethers, six organophosphate esters (OPEs), and two alternative brominated FRs. In nonhyperthyroid cats, serum free thyroxine (fT4), total T4 (TT4), total triiodothyronine, and thyroid-stimulating hormone concentrations were compared with FR concentrations. Tris(1,3-dichloro-2-isopropyl) phosphate (TDCIPP) concentrations were higher in hyperthyroid pet tags in comparison to nonhyperthyroid pet tags (adjusted odds ratio, p < 0.07; Mantel-Cox, p < 0.02). Higher TDCIPP concentrations were associated with air freshener use in comparison to no use (p < 0.01), residences built since 2005 compared to those pre-1989 (p < 0.002), and cats preferring to spend time on upholstered furniture in comparison to no preference (p < 0.05). Higher TDCIPP concentrations were associated with higher fT4 and TT4 concentrations (p < 0.05). This study provides proof-of-concept data for the use of silicone pet tags with companion animals and further indicates that bioavailable TDCIPP exposures are associated with feline hyperthyroidism.

Relationship between total thyroxine, thyroid palpation and a clinical index in hyperthyroid and healthy cats and cats with other diseases

OBJECTIVES

Present-day diagnosis of hyperthyroidism is often established in the early stages where clinical signs and physical examination findings typically associated with the disease may not yet be present. The purpose of this study was to investigate thyroid palpation score (TPS), total thyroxine (T4), body weight, body condition score and a clinical scoring index, which assesses severity of illness and quality of life, in untreated hyperthyroid cats, healthy cats and cats with non-thyroidal illness.

METHODS

Fifty-five cats with hyperthyroidism, 45 healthy cats and 327 euthyroid cats with non-thyroidal disease were prospectively enrolled. A Kruskal-Wallis test was used to determine any differences between metric data. A χ2 test was applied to compare nominal data between the three subgroups. Correlation between two variables was analysed using Spearman's correlation coefficient.

RESULTS

Almost 80% of the hyperthyroid cats and up to 20% of the healthy cats and cats with non-thyroidal illness had a palpable thyroid gland. Median TPS in hyperthyroid cats was 2, which was significantly higher compared with the other groups. Although there was a significant correlation between TPS and T4 when evaluating all cats, this could not be demonstrated in the three subgroups. Hyperthyroid cats with larger thyroid glands (TPS >3) did not have higher T4 levels. Hyperthyroid cats with lower clinical scores had significantly higher T4 levels compared with hyperthyroid cats with higher scores. Hyperthyroid cats were older and had lower body weights and body condition scores than other cats.

CONCLUSIONS AND RELEVANCE

Hyperthyroid cats were commonly found to have palpable thyroid glands, but they were smaller than previously reported.

Per- and polyfluoroalkyl substances in Northern California cats: Temporal comparison and a possible link to cat hyperthyroidism

The indoor environment and dietary intake are considered to be major human exposure pathways to per- and polyfluoroalkyl substances (PFASs). Cats have similar exposures to humans by sharing their residential environments, although they have different diet, body sizes, and indoor activities. In the present study, we report PFAS levels in the serum of 2 groups of Northern California cats (>10 yr old) collected during 2 time periods: 2008 to 2010 (n = 21) and 2012 to 2013 (n = 22). Levels of ∑PFAS (geometric mean) were lower in the second period (geometric mean = 8.10 ng/mL) than the first time period (geometric mean = 15.8 ng/mL), although PFAS profiles remained similar. We also analyzed PFAS levels in human serum collected in the same time period (2008-2010) and geographic area, and compared the profiles and ∑PFAS levels (15.8 vs 14.3 ng/mL for cat and human, respectively). Long chain perfluorinated carboxylic acids, especially perfluorononanoic acid and perfluoroundecanoic acid, were significantly higher in cat serum than in humans. Furthermore, serum from hyperthyroid cats in the second time period showed higher ∑PFAS level (9.50 ng/mL) compared to nonhyperthyroid cats (7.24 ng/mL), and it is the perfluorooctanoic acid levels that were statistically significantly higher in hyperthyroid cats' serum (p < 0.05). This result may indicate a possible link between PFAS levels and cat hyperthyroid, warranting a larger study for further investigation. Environ Toxicol Chem 2018;37:2523-2529. © 2018 SETAC.

Special Considerations for Diagnosing Behavior Problems in Older Pets

Aging is a complex process that involves changes in sensory perception, cognition, and physical strength. Physiologic, metabolic, and emotional changes of the aging process can lead to behavioral problems in older pets. Treatment of behavior problems in older pets requires a multimodal therapeutic approach. Frequent follow-up visits are required to monitor pets and comment on prognosis. Clients should be informed of the prognosis and should be educated to evaluate pain, discomfort, and general quality of life of the aging pet.

Prevalence and risk factors for hyperthyroidism in Irish cats from the greater Dublin area

Background

Hyperthyroidism is common in older cats. Prevalence varies geographically, but is anecdotally considered low in Ireland. The aim of this study was to document prevalence of hyperthyroidism in older cats in the greater Dublin area of Ireland and to assess environmental and clinical associations for development and identification of the disease.

Methods

Primary-care veterinary practices were requested to select cats aged 10 years or older where blood sampling was being performed for health screening or clinical investigations. Surplus serum/plasma samples were submitted to University College Dublin Diagnostic Endocrine Laboratory for total thyroxine (T₄) measurement. Cats were classified as hyperthyroid, equivocal or euthyroid based on a total T₄ concentration (reference interval, 15-60 nmol/L), of >60 nmol/L, 30-60 nmol/L or <30 nmol/L, respectively. Simultaneous free T₄ or repeat (after 4-6 weeks) total T₄ measurement was recommended in all equivocal cases. Animals receiving treatment for hyperthyroidism were excluded. A questionnaire completed by the client and veterinarian detailing historical and physical information was also required. Associations between categorical variables were analysed by Chi-square or Fisher's exact test and odds ratio (OR) calculated. A P value of <0.05 was considered statistically significant.

Results

Samples were submitted from 507 cats including 107 (21.1%) hyperthyroid, 54 (10.6%) equivocal and 346 (68.2%) euthyroid. The presence of goitre (P < 0.0001), tachypnoea (P = 0.0378), tachycardia (P = 0.002), polyphagia (P = 0.0003) and weight loss (P < 0.0001) were significantly associated with hyperthyroidism. Cats with goitre were more likely to be diagnosed as hyperthyroid [OR 2.85, (95% CI 1.75-4.62] compared to those without. However, goitre was only palpated in 40 of 102 (39.2%) hyperthyroid cats. Increasing age was the only significant (P < 0.002) risk factor for development of hyperthyroidism. A relationship between hyperthyroidism and sex, breed, lifestyle, parasite control, vaccination status or feeding habits was not identified.

Conclusions

Hyperthyroidism is not uncommon in Irish cats. Age was the only significant risk factor for its development. The high proportion of hyperthyroid cats without palpable goitre (> 60%) may reflect failure to detect goitre and account for the perceived low prevalence of this condition in Ireland.

Prävalenz und Risikofaktoren der felinen Hyperthyreose in einer Klinik population in Süddeutschland

Ziel: Die feline Hyperthyreose ist eine häufige Endokrinopathie bei älte ren Katzen. In früheren Studien wurden unausgewogene Ernährung, Schilddrüsen-Disruptoren, hohes Alter sowie fehlende Reinrassigkeit als Risikofaktoren diskutiert, ein endgültiger Auslöser bleibt jedoch unbekannt. Die Ziele dieser prospektiven Studie waren a) die Berech nung der Klinikprävalenz in einer Katzenpopulation in Süddeutschland, b) die Feststellung, wie häufig die Diagnose nach dem klini schem Verdacht bestätigt wurde und c) die Auswertung mutmaßlicher in- und extrinsischer Risikofaktoren anhand des Signalements der Katzen und eines Fragebogens. Methoden: Gesamt-Thyroxin (T4) wurde im Serum von 495 Katzen ≥ 8 Jahre gemessen und die Prävalenz mit einem 95%-Konfidenzintervall (95%-KI) berechnet. Abhängigkeiten zwischen Signalement und Hyperthyreose wurden durch den Student-T-Test, Chi-Square-Test und den Mann-Whitney U-Test analysiert. Das Signifikanzniveau lag bei 0,05. Zur Ermittlung extrinsischer Risikofaktoren diente ein logistisches Regressionsmodell. Ergebnisse: Bei 61 Katzen wurde eine Hyperthyreose diagnostiziert, was eine Prävalenz von 12,3% ergibt (95%-KI: 9,7–15,5). Ältere (p < 0,001) weibliche Katzen (p = 0,019; Odds Ratio 1,9) waren signifikant häufiger betroffen. Hauskatzen (Europäisch Kurz- und Langhaarkatzen) erkrankten häufiger als Rassekatzen (p = 0,016). Bei 164 Katzen wurde die Verdachtsdiagnose Hyperthyreose gestellt und in 20,1% (33/164) der Fälle verifiziert. In 2,4% (12/495) der Fälle war die erhöhte T4-Konzentration ein Zufallsbefund. Hyperthyreote Katzen wurden häufiger mit Nassfutter aus Aluminiumschalen (p < 0,013) gefüttert als nichthyperthyreote Katzen. Schlussfolgerung und klinische Re levanz: Ältere, weibliche Hauskatzen sind prädisponiert, an einer Schilddrüsenüberfunktion zu erkranken. Die Diagnose lässt sich häufig nach initialem klinischem Verdacht stellen, woraus in der Studienpopulation eine Prävalenz von 12,3% resultierte. Rückstände aus Aluminiumschalen oder das Nassfutter selbst scheinen eine Rolle in der Ätiopathogenese zu spielen.

Effects of dietary selenium and moisture on the physical activity and thyroid axis of cats

Consumption of canned cat food is considered a risk factor for the development of feline hyperthyroidism. Because selenium and water are substantially higher in canned diets compared to dry diets, objectives of this study were to determine whether increased dietary selenium or water alters the function of the hypothalamic-pituitary-thyroid axis and leads to an increase in activity level. Employing a 28-day latin square design with a 14-day washout, six lean, neutered male domestic shorthair cats were fed (i) commercially available adult dry feline diet containing 0.8 ppm selenium (control), (ii) control diet with added sodium selenite to achieve a dietary selenium concentration of 1.125 ppm (selenium treatment) and (iii) the control diet with additional water to achieve a moisture content of 75% wt/wt (water treatment). Water consumption was determined using deuterium oxide washout. Actical activity monitors were placed on each cat's collar to allow quantification of the activity of each cat. Circulating serum T3 and T4 was measured on days 0, 14, and 28. On day 28, a thyrotropin-releasing hormone (TRH) stimulation test was conducted to determine treatment effects on serum concentrations of thyroid hormones. There was a significant increase in daily water consumption with dietary water treatment (192 ml ± 7.85 SEM) compared to the control (120 ml ± 20.4) and selenium (116 ml ± 14.6) treatments. Both water and selenium treatments were associated with greater (p < .05) activity over that of the control treatment by 20.5% and 11% respectively. Serum TT3 AUC concentrations (0-4 hr) of TRH stimulation tests were greater (p < .05) by 16% with water compared to control treatments. The results of this study indicate that dietary water content may alter the function of the thyroid axis and that this effect is associated with an increase in physical activity.

Prevalence of and risk factors for feline hyperthyroidism in South Africa

Objectives Hyperthyroidism is a disorder of older cats that may have a geographical variation in prevalence. Prevalence studies have not yet been performed in South Africa, a geographical area where hyperthyroidism in cats has recently been observed and where, reportedly, the incidence appears to be increasing. The purpose of this study was to determine the prevalence of feline hyperthyroidism in South Africa and to identify any potential risk factors. Further information on the worldwide prevalence and possible causative factors would increase our understanding of the aetiology of this disease and help identify any preventive measures. Methods Serum total thyroxine (tT4) and canine thyroid-stimulating hormone (cTSH) were measured in 302 cats aged 9 years and older that were presented at various veterinary clinics throughout South Africa. In cats with equivocal tT4 and undetectable cTSH values, serum free thyroxine (fT4) was also measured. At the time of blood sampling a questionnaire was completed regarding vaccination history, internal and external parasite control, diet and environment. Results Prevalence of hyperthyroidism (tT4 >50 nmol/l or tT4 between 30 and 50 nmol/l with TSH <0.03 ng/ml and fT4 >50 pmol/l) was 7% (95% confidence interval 4.4-10.4), with no significant difference between healthy (5%) and sick (8%) cats. Cats ⩾12 years of age (odds ratio [OR] 4.3, P = 0.02) and cats eating canned food (OR 2.1, P = 0.1) were more likely to be diagnosed with hyperthyroidism. No significant relationship between vaccinations, parasite control or indoor environment and hyperthyroidism was observed. Hyperthyroid cats were more likely to present with weight loss (OR 3.2, P = 0.01) and with a heart rate ⩾200 beats per min (OR 5, P = 0.01) than cats without the disease. Conclusions and relevance Hyperthyroidism does not appear to be uncommon in the South African cat population. Risk factors for hyperthyroidism, specifically older age and eating canned food, were present in this as in other reported populations.

Breed, Coat Color, and Hair Length as Risk Factors for Hyperthyroidism in Cats

Background

Hyperthyroidism is very common in older cats, but the etiopathogenesis is poorly understood. Decreased risk of hyperthyroidism has been reported in certain colorpoint breeds, and this observation previously has been hypothesized to result from relatively greater tyrosine availability for thyroid hormone production because of limited ability to convert tyrosine to melanin pigment. However, studies investigating a potential link between coat pigmentation and risk of hyperthyroidism are limited.

Objective

To identify associations between coat phenotype and hyperthyroidism by investigation of breed, coat color, and hair length as risk factors for the disease.

Animals

Data were used from 4,705 cats aged ≥10 years, referred to a single veterinary teaching hospital (2006–2014) in the United Kingdom.

Methods

Retrospective, epidemiological, cross‐sectional study using Bayesian multivariable logistic regression to assess risk factors for hyperthyroidism.

Results

Burmese (odds ratio [OR], 0.01; 0.00–0.23; P = .004), Tonkinese (OR, 0.05; 0.00–0.95; P = .046), Persian (OR, 0.21; 0.10–0.44; P < .001), Siamese (OR, 0.27; 0.12–0.61; P = .002), Abyssinian (OR, 0.04; 0.00–0.74; P = .031), and British shorthair (OR, 0.47; 0.28–0.79; P = .004) breeds had decreased risk of hyperthyroidism compared to domestic shorthairs. Longhaired, nonpurebred cats (OR, 1.30; 1.03–1.64; P = .028) were at increased risk of hyperthyroidism. Coat color/pattern was not associated with hyperthyroidism in nonpurebred cats.

Conclusions and Clinical Importance

We identified decreased risk of hyperthyroidism in the Tonkinese, Abyssinian, and British shorthair breeds, identified an association between risk of hyperthyroidism and hair length, and confirmed decreased risk in Burmese, Siamese, and Persian breeds. Additional studies are warranted to further investigate these findings.

Association of Polybrominated Diphenyl Ethers (PBDEs) and Polychlorinated Biphenyls (PCBs) with Hyperthyroidism in Domestic Felines, Sentinels for Thyroid Hormone Disruption

BACKGROUND

Hyperthyroidism is the most common endocrine disorder observed in domestic felines; however, its etiology is largely unknown. Two classes of persistent organic pollutants, polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are known to interfere with thyroid hormone (TH) signaling and regulation; thus, it is postulated that they contribute to the etiopathogenesis of feline hyperthyroidism and pose a risk to humans and other species. In this case-control study, the concentrations of 13 PBDE and 11 PCB congeners were measured by gas chromatography mass spectrometry in serum or plasma samples from 20 hyperthyroid and 31 control cats in order to investigate the association between concentration of PBDE and PCB congeners and feline hyperthyroidism. Logistic regression analysis was used to determine whether elevated concentrations of individual congeners were associated with a higher risk of feline hyperthyroidism.

RESULTS

Hyperthyroid cats had higher concentrations of four PBDE congeners (BDE17, BDE100, BDE47, and BDE49) and five PCB congeners (PCB131, PCB153, PCB174, PCB180, and PCB196), compared to control cats. In addition, the sum of both PBDE and PCB congener concentrations were elevated in the hyperthyroid group compared to control cats; however, only the increased PCB concentrations were statistically significant. The sum total PBDE concentrations in our feline samples were approximately 50 times greater than concentrations previously reported in human populations from a geographically similar area, whereas sum total PCB concentrations were comparable to those previously reported in humans.

CONCLUSIONS

These observational findings support the hypothesis that PBDEs and PCBs may contribute to the etiopathogenesis of hyperthyroidism in felines. As domestic house cats are often exposed to higher concentrations of PBDEs than humans, they may serve as sentinels for the risk of TH disruption that these pollutants pose to humans and other species.

Dietary Management of Hyperthyroidism in a Dog

An 8 yr old female spayed golden retriever presented for a routine exam during which ventral cervical soft tissue masses were identified. History included weight loss, increased activity and appetite, gagging, and occasional diarrhea. Exam findings included a body condition score of 4/9 and palpable ventral cervical nodules. A serum thyroxine (T4) value was 8.0 ug/dL (normal = 0.8-3.5ug/dL). Doppler systolic blood pressure readings ranged from 200-210 mmHg (normal systolic blood pressure <150 mmHg). The diagnosis was hyperthyroidism due to active thyroid masses. Due to financial constraints, the owner elected conservative management. Initial treatment with methimazole resulted in a decreased T4 value of 5.0 ug/dL at approximately 4 mo after initiation of treatment. A commercially available iodine-restricted feline diet was fed and this resulted in further reduction in serum T4 levels, improved sleeping cycles, reduced anxiety, and reduced systolic blood pressure. A temporary suspension of iodine-restricted feline diet for 2 mo resulted in increases in serum T4 concentrations, which, subsequently, decreased with re-introduction of the diet. Roughly 10 mo after initiation of the therapeutic diet and 16 mo after intial diagnosis, the dog remains relatively normal clinically despite active growing cervical masses with T4 concentration of 2.3 ug/dL.

2016 AAFP Guidelines for the Management of Feline Hyperthyroidism

CLINICAL CONTEXT

Since 1979 and 1980 when the first reports of clinical feline hyperthyroidism (FHT) appeared in the literature, our understanding of the disease has evolved tremendously. Initially, FHT was a disease that only referral clinicians treated. Now it is a disease that primary clinicians routinely manage. Inclusion of the measurement of total thyroxine concentration in senior wellness panels, as well as in diagnostic work-ups for sick cats, now enables diagnosis of the condition long before the cat becomes the classic scrawny, unkempt, agitated patient with a bulge in its neck. However, earlier recognition of the problem has given rise to several related questions: how to recognize the health significance of the early presentations of the disease; how early to treat the disease; whether to treat FHT when comorbid conditions are present; and how to manage comorbid conditions such as chronic kidney disease and cardiac disease with treatment of FHT. The 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism (hereafter referred to as the Guidelines) will shed light on these questions for the general practitioner and suggest when referral may benefit the cat.

SCOPE

The Guidelines explain FHT as a primary disease process with compounding factors, and provide a concise explanation of what we know to be true about the etiology and pathogenesis of the disease.The Guidelines also:Distill the current research literature into simple recommendations for testing sequences that will avoid misdiagnosis and separate an FHT diagnosis into six clinical categories with associated management strategies.Emphasize the importance of treating all hyperthyroid cats, regardless of comorbidities, and outline the currently available treatments for the disease.Explain how to monitor the treated cat to help avoid exacerbating comorbid diseases.Dispel some of the myths surrounding certain aspects of FHT and replace them with an evidence-based narrative that veterinarians and their practice teams can apply to feline patients and communicate to their owners.

EVIDENCE BASE

To help ensure better case outcomes, the Guidelines reflect currently available, evidenced-based knowledge. If research is lacking, or if a consensus does not exist, the expert panel of authors has made recommendations based on their extensive, cumulative clinical experience.

Temporal Changes of PBDE Levels in California House Cats and a Link to Cat Hyperthyroidism

In this study, we measured serum PBDE levels in California (CA) house cats during two time periods: 2008-2010 and 2012-2013 to assess the impacts of the decline in use of these materials after the bans. The median ∑19PBDE level in CA household cats (age ≥10 yr) was 3479 ng/g lipid in 2008-2010 (1st time period, n = 21) and 1518 ng/g lipid in 2012-2013 (2nd time period, n = 22), about 2 times lower than in the first time period (p = 0.006). In contrast, PCB and OCP levels showed no statistically significant changes. With better matched group size and age (HT = 11 vs non-HT = 11, age ≥10 yr) in the second time period, we found that ∑19PBDE level (mean ± SE ng/g lipid) was significantly higher in the HT group (3906 ± 1442) than those in the non-HT group (1125 ± 244) (p = 0.0030). Higher levels of PCBs and OCPs were also found in HT group. Despite the declines of PBDE levels, our findings indicate that the current levels of PBDEs, as well as PCBs and OCPs, may still pose health effects for house cats and, possibly, humans.

Soy isoflavone metabolism in cats compared with other species: urinary metabolite concentrations and glucuronidation by liver microsomes

1. Soybean is a common source of protein in many pet foods. Slow glucuronidation of soy-derived isoflavones in cats has been hypothesized to result in accumulation with adverse health consequences. Here, we evaluated species' differences in soy isoflavone glucuronidation using urine samples from cats and dogs fed a soy-based diet and liver microsomes from cats compared with microsomes from 12 other species. 2. Significant concentrations of conjugated (but not unconjugated) genistein, daidzein and glycitein, and the gut microbiome metabolites, dihydrogenistein and dihydrodaidzein, were found in cat and dog urine samples. Substantial amounts of conjugated equol were also found in cat urine but not in dog urine. 3. β-Glucuronidase treatment showed that all these compounds were significantly glucuronidated in dog urine while only daidzein (11%) and glycitein (37%) showed any glucuronidation in cat urine suggesting that alternate metabolic pathways including sulfation predominate in cats. 4. Glucuronidation rates of genistein, daidzein and equol by cat livers were consistently ranked within the lowest 3 out of 13 species' livers evaluated. Ferret and mongoose livers were also ranked in the lowest four species. 5. Our results demonstrate that glucuronidation is a minor pathway for soy isoflavone metabolism in cats compared with most other species.

Higher PBDE serum concentrations may be associated with feline hyperthyroidism in Swedish cats

Serum from 82 individual cats was analyzed for decabromobiphenyl (BB-209), polybrominated diphenyl ethers (PBDEs), hydroxylated PBDEs (OH-PBDEs), and 2,4,6-TBP in order to study differences in body burden between healthy and sick cats diagnosed with Feline Hyperthyroidism (FH). Within the study group, 60 of these cats had a euthyroid (n = 23) or hyperthyroid (n = 37) status, all of which were used in the comparison. This study shows that hyperthyroid compared to euthyroid cats have higher serum concentrations for some of the investigated PBDEs (BDE-99, BDE-153, and BDE-183) and CB-153 on a fat weight basis. Further, it is intriguing, and beyond explanation, why the flame retardant BB-209 (discontinued in 2000) is present in all of the cat serum samples in concentrations similar to BDE-209. Median BDE-47/-99 ratios are 0.47 and 0.32 for healthy and euthyroid cats, respectively, which differs significantly from Swedes, where the ratio is 3.5. Another important finding is the occurrence of very low levels or the absence of hydroxylated PBDE metabolites in the cats. In addition, the major OH-PBDE, 6-OH-BDE47, is likely of natural origin, probably ingested via cat food. The statistics indicate an association between elevated PBDE concentrations in the cats and FH.

Evaluation of polybrominated diphenyl ethers (PBDEs) in matched cat sera and house dust samples: investigation of a potential link between PBDEs and spontaneous feline hyperthyroidism

The cause of feline hyperthyroidism (FH), a common endocrinopathy of domestic cats, is unknown. A potential association between exposure to environmental contaminants polybrominated diphenyl ethers (PBDEs) and FH was investigated. The median serum level for the sum of congeners BDE-47, BDE-99, BDE-153, BDE-154 and BDE-183 (Σ5) in hyperthyroid and euthyroid cats was 82 and 174 ng g(-1)lw respectively with no significant difference in PBDE levels or profiles between groups. Overall, the median (min to max) concentration of PBDEs in cat serum (n=65) was 118 ng g(-1)lw (5-5260 ng g(-1)lw), which is approximately 10 times higher than that observed in the Australian human population. Furthermore, congener composition in feline serum samples was dominated by congener BDE-99, followed by BDE-47 then BDE-153 which differs from results of human biomonitoring. There was no correlation between PBDE levels in feline serum samples and matched house dust samples (n=25). However the similarity of BDE-47/99 ratio in each matrix suggests dust is likely the dominant exposure. Calculation of the daily exposure dose via dust ingestion for cats equated to a mean of 33 ng kg(-1) bw d(-1) (0.2-150 ng kg(-1) bw d(-1)). Differences in exposure estimates for Australian and US cats, based on dust ingestion alone, are consistent with the observed differences in body burdens. Our results do not support a role for PBDE exposure in the aetiopathogenesis of FH.

A critical review of food-associated factors proposed in the etiology of feline hyperthyroidism

Since the first description of feline hyperthyroidism (HT) in 1979, several studies have been undertaken to define the etiology of the disease. Epidemiologic studies, after investigating non-food- and food-associated factors, suggest a multifactorial etiology. However, in the absence of prospective cohort studies that can confirm a cause-and-effect relationship between HT and associated risk factors, no causative factor for HT has been identified to date. Feline HT resembles toxic nodular goiter in humans, with autonomously functioning upregulated iodide uptake systems. Contribution of the diet to HT development remains controversial. The purpose of this paper is to review critically the reported food-associated risk factors for HT.

Animal models of disease: feline hyperthyroidism: an animal model for toxic nodular goiter

Since first discovered just 35 years ago, the incidence of spontaneous feline hyperthyroidism has increased dramatically to the extent that it is now one of the most common disorders seen in middle-aged to senior domestic cats. Hyperthyroid cat goiters contain single or multiple autonomously (i.e. TSH-independent) functioning and growing thyroid nodules. Thus, hyperthyroidism in cats is clinically and histologically similar to toxic nodular goiter in humans. The disease in cats is mechanistically different from Graves' disease, because neither the hyperfunction nor growth of these nodules depends on extrathyroidal circulating stimulators. The basic lesion appears to be an excessive intrinsic growth capacity of some thyroid cells, but iodine deficiency, other nutritional goitrogens, or environmental disruptors may play a role in the disease pathogenesis. Clinical features of feline toxic nodular goiter include one or more palpable thyroid nodules, together with signs of hyperthyroidism (e.g. weight loss despite an increased appetite). Diagnosis of feline hyperthyroidism is confirmed by finding the increased serum concentrations of thyroxine and triiodothyronine, undetectable serum TSH concentrations, or increased thyroid uptake of radioiodine. Thyroid scintigraphy demonstrates a heterogeneous pattern of increased radionuclide uptake, most commonly into both thyroid lobes. Treatment options for toxic nodular goiter in cats are similar to that used in humans and include surgical thyroidectomy, radioiodine, and antithyroid drugs. Most authorities agree that ablative therapy with radioiodine is the treatment of choice for most cats with toxic nodular goiter, because the animals are older, and the disease will never go into remission.

Feline hyperthyroidism reported in primary-care veterinary practices in England: prevalence, associated factors and spatial distribution

Feline hyperthyroidism is a commonly diagnosed endocrinopathy that can have a substantial deleterious impact on the welfare of affected cats. This study aimed to estimate the prevalence, associated factors and geographical distribution for feline hyperthyroidism in England, using primary-care veterinary practice clinical data from the VetCompass Animal Surveillance Project. Prevalence was estimated from the overall cat cohort. Associated factor analysis used an age-matched, nested, case-control design with multivariable logistic regression. There were 2,276 cases of feline hyperthyroidism identified from 95,629 cats attending 84 practices from September 2009 to December 2011. Cases were aged 6-25 years. 3.7 per cent of cases and 9.9 per cent of controls were purebred, 56.4 per cent of cases and 56.5 per cent of controls were female, and 88.1 per cent of cases and 86.0 per cent of controls were neutered. The apparent prevalence was 2.4 per cent (95% CI 2.3 to 2.5 per cent) overall, and 8.7 per cent (95% CI 8.3 to 9.0 per cent) in cats aged 10 years or above. Burmese (OR 0.15, 95% CI 0.07 to 0.32, P<0.0001), Persian (OR 0.17, 95% CI 0.08 to 0.33, P<0.0001), Siamese (OR 0.4, 95% CI 0.21 to 0.75, P=0.004) and purebred cats overall (OR 0.33, 95% CI 0.25 to 0.42, P< 0.0001) had lower odds of feline hyperthyroidism than non-purebred cats. Insured cats had increased odds (OR 1.78, 95% CI 1.56 to 2.03, P< 0.001). There was little evidence of spatial variation. This study highlights feline hyperthyroidism as a high-prevalence disease in England, and reports reduced odds of diagnosis in certain breeds and purebred cats overall.

Worldwide prevalence and risk factors for feline hyperthyroidism: A review

Since first reported in the late 1970s, there has been a steady but dramatic increase in the worldwide prevalence of hyperthyroidism in cats. It is now regarded as the most common feline endocrine disorder, with diabetes mellitus coming a close second. Not only is there evidence for an increased worldwide prevalence of feline hyperthyroidism, but also for geographical variation in the prevalence of the disease. Despite its frequency, the underlying cause(s) of this common disease is or are not known, and therefore prevention of the disease is not possible. Due to the multiple risk factors that have been described for feline hyperthyroidism, however, it is likely that more than one factor is involved in its pathogenesis. Continuous, lifelong exposure to environmental thyroid-disruptor chemicals or goitrogens in food or water, acting together or in an additive fashion, may lead to euthyroid goitre and ultimately to autonomous adenomatous hyperplasia, thyroid adenoma and hyperthyroidism. This review aims to summarise the available published evidence for the changes observed in the worldwide prevalence of the disease, as well as risk factors that may contribute to development of hyperthyroidism in susceptible cats.

Iodine concentration in commercial cat foods from three regions of the USA, 2008–2009

Fluctuations in iodine concentration in food have been suggested as one risk factor for the development of feline hyperthyroidism, an epidemic disease first described in 1979. Three international studies have examined iodine concentrations of commercial cat foods. The iodine concentration of 112 commercial cat foods from across the USA was measured, and the daily iodine intake by hypothetical 4.5 kg adult cats or 1.4 kg kittens calculated in this descriptive epidemiologic study to examine differences in feline iodine intake due to (i) geographical source of foods, (ii) packaging type, (iii) brand-to-brand variation, (iv) form of iodine supplementation, (v) types and numbers of seafood ingredients and (vi) kitten and ‘therapeutic’ diets. Dramatic variation among canned foods (resulting in ingestion of approximately 49–9639 μg iodine/day) suggests that the disparity in iodine concentrations may lead to development of nodular hyperplasia and, later, clinical hyperthyroidism, if cats consume diets that are at first iodine-deficient and later contain excessive iodine. Manufacturers are encouraged to ensure adequate iodine supplementation across all products and areas of the USA.

Hyperthyroidism in Cats

Practical relevance:

Since first being reported in the late 1970s, there has been a dramatic increase in the prevalence of hyperthyroidism in cats. It is now recognized worldwide as the most common feline endocrine disorder.

Patient group:

Hyperthyroidism is an important cause of morbidity in cats older than 10 years of age. It is estimated that over 10% of all senior cats will develop the disorder.

Clinical challenges:

Despite its frequency, the underlying cause(s) of this common disease is/are not known, and no one has suggested a means to prevent the disorder. Because of the multiple risk factors that have been described for feline hyperthyroidism, it is likely that more than one factor is involved in its pathogenesis. Continuous, lifelong exposure to environmental thyroid disruptor chemicals or goitrogens in food or water, acting together in an additive or synergistic manner, may first lead to euthyroid goiter and then to autonomous adenomatous hyperplasia, thyroid adenoma and hyperthyroidism.

Evidence base:

This review draws on published research studies to summarize the available evidence about the risk factors for feline hyperthyroidism. Based on the known goitrogens that may be present in the cat’s food, drinking water or environment, it proposes measures that cat owners can implement that might prevent, or reduce the prevalence of, thyroid tumors and hyperthyroidism in their cats.

High polybrominated diphenyl ether levels in California house cats: house dust a primary source?

Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants that act as endocrine disruptors, affecting thyroid hormone homeostasis. As a follow-up to a recent study showing high PBDE levels in household cats and linking PBDE levels with cat hyperthyroidism, we measured PBDEs, polychlorinated biphenyls (PCBs), and organochlorinated pesticides (OCPs) in serum samples from 26 California household cats (16 hyperthyroid, 10 controls) using liquid-liquid extraction and high-resolution gas chromatography/high-resolution mass spectrometry. In the present pilot study, we found that PBDE levels in California house cats were extremely high (ΣPBDEs median = 2,904 ng/g lipid; range, 631-22,537 ng/g lipid). This is approximately 50 times higher than levels in California residents (ΣPBDEs geomean = 62 ± 8.9 ng/g lipid, National Health and Nutrition Examination Survey), who have among the highest human levels in the world. Polybrominated diphenyl ethers congener patterns (BDE-99 major congener, BDE-209 significant) differed markedly from patterns found in California residents (BDE-47 major) or wildlife but resembled patterns found in house dust. Polychlorinated biphenyls and OCPs in cats were highly correlated, consistent with a shared dietary source or pathway of exposure, but did not correlate with PBDEs. This suggests a different source or pathway of exposure for PBDEs, which was most likely house dust. The authors found no evidence that linked levels of PBDEs, PCBs, or OCPs with hyperthyroidism. This may be because of the small sample size, competing or confounding risk factors, or complicated causal mechanisms.

Feline Hyperthyroidism: Potential Relationship with Iodine Supplement Requirements of Commercial Cat Foods

Article rationale Since the late 1970s, there has been a significant increase in the prevalence of feline hyperthyroidism (FH). It is now recognized worldwide as the most common endocrinopathy of older cats, resembling toxic nodular goiter of older humans in iodine-deficient areas. The purpose of this article is to identify the potential for iodine concentrations in the diet to contribute to the etiology of FH.

Historical context Iodine concentrations of commercial cat foods vary widely. A review of historical iodine recommendations revealed that the units of iodine supplementation changed in the 1970s. Given this change, foods minimally supplemented since the late 1970s would have been iodine deficient for most cats.

Practical relevance Iodine supplementation of commercial cat foods should be evaluated in the light of the iodine recommendations revised in 2006. Foods may remain deficient in iodine if supplemented at the minimum recommended concentration, possibly contributing to the development of FH.

Multiple endocrine diseases in cats: 15 cases (1997–2008)

The objective of this retrospective study was to characterize a population of cats from a tertiary care center diagnosed with multiple endocrine disorders, including the specific disorders and time intervals between diagnosis of each disorder. Medical records of 15 cats diagnosed with more than one endocrine disorder were reviewed. The majority of cats were domestic shorthairs, and the mean age at the time of diagnosis of the first disorder was 10.3 years. The most common combination of disorders was diabetes mellitus and hyperthyroidism. Two cats had concurrent diabetes mellitus and hyperadrenocorticism, one cat had concurrent central diabetes insipidus and diabetes mellitus. A mean of 25.7 months elapsed between diagnoses of the first and second endocrine disorder, but this was variable. This study suggests the occurrence of multiple endocrine disorders is uncommon in cats.

Evaluation of activation of G proteins in response to thyroid stimulating hormone in thyroid gland cells from euthyroid and hyperthyroid cats

OBJECTIVE

To evaluate alterations in ligand-stimulated activity of G proteins in thyroid gland cells of hyperthyroid cats.

SAMPLE POPULATION

Membranes of thyroid gland cells isolated from 5 hyperthyroid cats and 3 age-matched euthyroid (control) cats immediately after the cats were euthanatized.

PROCEDURES

Isolated thyroid cell membranes were treated with thyroid-stimulating hormone (TSH), and activation of G protein was quantified by measurement of the binding of guanosine triphosphate gamma labeled with sulfur 35 (GTPgamma(35)S). The separate effects of G-protein inhibitory (G(i)) and G-protein stimulatory (G(s)) proteins were determined by the use of pertussis toxin and cholera toxin, respectively.

RESULTS

Thyroid cell membranes from hyperthyroid cats had higher basal GTPgamma(35)S binding than did thyroid cell membranes from euthyroid cats. Thyroid cell membranes from hyperthyroid and euthyroid cats had a concentration-dependent increase in TSH-stimulated GTPgamma(35)S binding over the TSH range of 0 to 100 mU/mL, with maximal activity at 1 to 100 mU/mL for both. The percentage increase in GTPgamma(35)S binding stimulated by TSH was similar in magnitude between the membranes from hyperthyroid and euthyroid cats. The TSH-stimulated activation of G(s) and G(i) was not different between euthyroid and hyperthyroid cats.

CONCLUSIONS AND CLINICAL RELEVANCE

Ligand-stimulated activation of G proteins was the same in thyroid cell membranes obtained from hyperthyroid and euthyroid cats. Therefore, alterations in inherent G(s) or G(i) activities did not appear to be part of the pathogenesis of hyperthyroidism in cats.

Urinary iodide concentration in hyperthyroid cats

OBJECTIVE

To compare concentrations of urinary iodide (UI) in euthyroid and untreated hyperthyroid cats.

ANIMALS

118 euthyroid and 88 hyperthyroid client-owned cats from 2 nonreferral veterinary practices.

PROCEDURES

Iodide concentration was measured in 5 urine samples collected every 3 to 12 months from selected cats, and variability of results between euthyroid cats and hyperthyroid cats prior to the diagnosis of hyperthyroidism was evaluated via 1-way ANOVA, after logarithmic transformation of UI concentrations (logUIs). The UI concentration in hyperthyroid cats was measured at diagnosis and 2 to 6 weeks and 3 to 6 months after treatment for hyperthyroidism. The pretreatment logUI in hyperthyroid cats was compared with that in euthyroid cats, taking into account the effects of renal function on UI concentration. Iodine intake was estimated in euthyroid cats following calculation of the volume of daily urine output, with a fixed value for iodine concentration in feces.

RESULTS

The variability of UI concentrations did not differ significantly between hyperthyroid (n = 10) and euthyroid (8) cats. The logUI increased 2 to 6 weeks after initiation of treatment in hyperthyroid cats (n = 80) and was lower in azotemic versus nonazotemic cats. Hyperthyroid cats had a lower logUI than euthyroid cats, and there was no evidence of deficient iodine intake in euthyroid cats.

CONCLUSIONS AND CLINICAL RELEVANCE

The logUI was lower in cats with azotemia and with untreated hyperthyroidism, compared with that in euthyroid cats from the same population. Additional studies are needed to determine whether iodine intake plays a role in the development of hyperthyroidism in cats.

Prevalence of and risk factors for feline hyperthyroidism in Hong Kong

A study was conducted to determine the prevalence of and potential risk factors for feline hyperthyroidism in Hong Kong. Serum total thyroxine (T4) was measured in 305 cats aged 10 years and older that presented at various veterinary clinics in Hong Kong. The prevalence of hyperthyroidism (T4>50 nmol/l) within this population was 3.93% and there was no significant difference in prevalence between healthy (3.16%) and sick (4.37%) cats. Older cats (>15 years) were more likely to be affected and domestic shorthair cats were less likely to be diagnosed with hyperthyroidism than the other breeds combined. No specific association between the development of feline hyperthyroidism and food type was observed. The prevalence of feline hyperthyroidism in Hong Kong was less than that reported for most other parts of the world, despite the presence of previously identified risk factors.

Etiopathologic Findings of Hyperthyroidism in Cats

None of the studies to date have isolated a single dominant factor that could be incriminated in the development of hyperthyroidism in cats. Rather, most of the studies provide further evidence of the widely held view that hyperthyroidism is a multifactorial disease in this species. At this time, the most likely candidates include one or more of the goitrogenic chemicals that have been shown to be present in cat food or the cat's environment. In addition, mutations of the thyroid stimulating hormone receptor gene or mutations of its associated G proteins seem to play an important role in the pathogenesis of this disease.

Thyroid scintigraphy in hyperthyroidism

Thyroid scintigraphy is a nuclear medicine procedure that produces a visual display of functional thyroid tissue based on the selective uptake of various radionuclides by thyroid tissue. Thyroid scintigraphy provides valuable information regarding both thyroid anatomy and physiology and can play an integral role in the diagnosis and management of cats with hyperthyroidism. Thyroid scintigraphy allows the direct visualization of the functional adenomatous thyroid tissue responsible for the development of hyperthyroidism. For this reason, thyroid scintigraphy will allow the diagnosis of hyperthyroidism before laboratory tests are consistently abnormal. Thyroid scintigraphy can also exclude a diagnosis of hyperthyroidism in cats with thyroid hormone elevations of nonthyroidal origin. Thyroid scintigraphy provides an additional method for determining the relative severity of thyroid disease that is less affected by the presence of concurrent nonthyroidal illness than laboratory evaluations. When treating hyperthyroid cats with radioiodine, the lowest effective dose should be administered. In an effort to administer the lowest radioiodine dose possible, the volume of adenomatous thyroid tissue present in the individual hyperthyroid cat should be considered. Thyroid scintigraphy provides an excellent method for evaluating the size of hyperfunctional thyroid tissue that is not limited by the presence of ectopic or intrathoracic thyroid tissue. Thyroid scintigraphy also provides valuable information in the diagnosis and evaluation of hyperthyroid cats with thyroid carcinoma.

Expression and purification of feline thyrotropin (fTSH): immunological detection and bioactivity of heterodimeric and yoked glycoproteins

The goal of this study was to express and purify recombinant feline TSH as a possible immunoassay standard or pharmaceutical agent. Previously cloned feline common glycoprotein alpha (CGA) and beta subunits were ligated into the mammalian expression vector pEAK10. The feline CGA-FLAG and beta subunits were cloned separately into the pEAK10 expression vector, and transiently co-transfected into PEAK cells. Similarly, previously cloned and sequenced yoked (single chain) fTSH (yfTSH) and the CGA-FLAG sequences were ligated into the same vector, and stable cell lines selected by puromycin resistance. Expression levels of at least 1 microg/ml were achieved for both heterodimeric and yoked fTSH forms. The glycoproteins were purified in one step using anti-FLAG immunoaffinity column chromatography to high purity. The molecular weights of feline CGA-FLAG subunit, beta subunit and yfTSH were 20.4, 17, and 45 kDa, respectively. Both heterodimeric and yoked glycoproteins were recognized with approximately 40% detection by both a commercial canine TSH immunoassay and an in-house canine TSH ELISA. The yoked glycoprotein exhibited parallelism with the heterodimeric form in the in-house ELISA, supporting their possible use as immunoassay standards. In bioactivity assays, the heterodimeric and yoked forms of fTSH were 12.5 and 3.4% as potent as pituitary source bovine TSH at displacing (125)I-bTSH and 45 and 24% as potent in stimulating adenylate cyclase activity in human TSH receptor-expressing JP09 cells. However, in addition to reduced receptor binding affinity, the recombinant glycohormones produced a reduced maximal effect at maximal concentration (E(max)) suggesting the possibility of the recombinant glycohormone constructs acting as partial agonists at the human TSH receptor.

Thyroid Disorders in the Geriatric Patient

Thyroid disorders are common in older pets. They often present a diagnostic challenge, and reaching a definitive diagnosis can be difficult or impossible in some cases. It is important for the veterinary practitioner to be familiar with the historical, physical examination, and clinicopathologic data findings in each of these diseases and to become comfortable with the treatment, monitoring, and prognosis associated with thyroid diseases in geriatric pets.

Multivariate analysis of risk factors for feline hyperthyroidism in New Zealand

AIM

To search for putative risk factors for feline hyperthyroidism in New Zealand, using a case-control study.

METHODS

A questionnaire-based case-control study involving the owners of 375 cats in New Zealand (125 hyperthyroid cats, 125 randomly selected control cats, and 125 age- and sex-matched control cats) was conducted to examine associations between potential risk factors and occurrence of feline hyperthyroidism. Data were collected between December 1996 and February 1998, relative to cat and owner demography and medical history, cats' indoor and outdoor environments, and cats' diets. A range of statistical techniques was employed to analyse the data, including descriptive analyses, univariate logistic regression for each variable and multivariate stepwise forward logistic regression.

RESULTS

Multivariate analysis revealed that affected cats were more likely to be female (odds ratio (OR)=3.3; 95% confidence interval (CI)=1.2-9.0) and older than unaffected random control cats. Purebred cats were at a much lower risk of being diagnosed as hyperthyroid than were domestic short- and long-haired cats (OR=0.01; 95% CI=0.001-0.20). If more than one cat was present in a household, hyperthyroidism was less likely to be identified (OR=0.15; 95% CI=0.05-0.44) compared with single-cat households. Hyperthyroid cats were 6.6 times more likely (95% CI=1.8-23.9) to be reported to sleep predominantly on the floor than control cats. Cats whose bedding was regularly treated with anti-flea products appeared to be at a considerably higher risk for hyperthyroidism (OR=57.6; 95% CI=3.8-->200); and, to a lesser extent, so were cats living in households where fly sprays were reported to be used regularly (OR=3.3; 95% CI=1.2-9.3). The interaction between drinking water from puddles and regular use of organic garden fertilisers, such as compost or animal manure, was associated with a 5.3-fold (95% CI=1.1-25.6) increase in the risk of cats being diagnosed with the disease. Hyperthyroid cats were twice as likely (95% CI=0.3-12.9) to have eaten at least half of their daily food requirements as canned commercial cat food compared with unaffected cats. Cats exposed to a variety of flavours of canned cat food were more likely to be diagnosed with hyperthyroidism than were those fed only one flavour (OR=3.8; 95% CI=1.5-9.6). The presence of dental disorders was associated with a 5.5-fold increase in the risk of being diagnosed as hyperthyroid and this association was independent of the cat's age (95% CI=1.7-17.5).

CONCLUSIONS

The results of this study support and extend those in several earlier reports and show that cats in New Zealand are, in many respects, similar to cats in Europe and North America in terms of their susceptibility to hyperthyroidism. The finding that female cats are predisposed to hyperthyroidism is at variance with most previously published work. It remains unclear which, if any, of the identified disease associations are causal, so further studies of this increasingly prevalent feline endocrinopathy are warranted.

Feline endocrinopathies

Feline endocrinopathies (excluding diabetes mellitus) include hyperthyroidism, hypothyroidism, acromegaly, hyposomatotropism, diabetes insipidus, hyperadrenocorticism, primary sex hormone-secreting adrenal tumors, primary hyperaldosteronism, pheochromocytoma, hypoadrenocorticism, hyperparathyroidism, and hypoparathyroidism. Each of these conditions will be discussed including their prevalence, cause, clinical signs, diagnosis, treatment options, and prognosis.

Juvenile hyperthyroidism in a cat

An 8-month-old, male domestic shorthaired cat presented for chronic weight loss, intermittent dyspnea, chronic diarrhea, hyperactivity, and weakness. The cat had a palpable thyroid nodule and increased serum total thyroxine and 3,5,3' triiodothyronine levels. The cat was diagnosed with hyperthyroidism, and a unilateral thyroidectomy was performed followed by radioactive iodine at a later date. The clinical signs resolved following radioactive iodine, and the cat subsequently developed clinical hypothyroidism.

Relationship between semi-quantitative thyroid palpation and total thyroxine concentration in cats with and without hyperthyroidism

In 155 cats, both with and without clinical signs of hyperthyroidism, total thyroxine (TT4) concentrations were compared to a sensitive, semi-quantitative thyroid palpation technique. On the basis of TT4 concentrations, 23 of the 155 cats were classified as hyperthyroid. The size of individual thyroid glands was scored between '0' (non-palpable) and a maximum of '6'. One or more enlarged thyroid glands (score >0) were palpated in 22 of the 23 hyperthyroid cats and in 78 of the 132 euthyroid cats. However, none of the 132 euthyroid cats had a thyroid lobe score of greater than '3' whereas 18 of the 23 hyperthyroid cats had a thyroid lobe score of '4' or greater, and in two of the five that had scores below '4' there was evidence of intrathoracic functional thyroid tissue on scintigraphy.

Feline endocrinology update

This article highlights the advances in feline endocrinology, excluding diabetes mellitus and hyperadrenocorticism, which have recently been reviewed elsewhere. The goal will be to provide clinically relevant information regarding pathogenesis, diagnosis, and treatment options for these feline endocrine disorders.