Best practice for the pharmacological management of hyperthyroid cats with antithyroid drugs

Radioiodine treatment in hyperthyroid cats: insights into the characteristics of owners and their cats, and owner motivation and perceptions

OBJECTIVES

Radioiodine (131I) therapy is the most appropriate treatment option for many hyperthyroid cats, as it is minimally invasive and often curative. Nevertheless, 131I treatment is not always pursued by owners. Hence, it is important to obtain more insight into owner satisfaction during and after 131I treatment, and their decision-making process. In this study, we describe the characteristics of owners and their hyperthyroid cats referred for 131I therapy, and determine owners' motivation and how they experienced the 131I treatment of their cat.

METHODS

A survey was sent to owners whose cats underwent 131I therapy (n = 1071) between 2010 and 2017 at Ghent University. The survey contained 35 questions with tick-box or free-text answer options concerning family situation, pet insurance, previous therapy, comorbidities, motivation for 131I therapy and owner perception of this treatment.

RESULTS

In total, 438 owners completed 94% or more of the questionnaire. Over half of the cats (55%) had received previous medical, dietary or surgical treatment. Motivations for changing the initial therapy to 131I therapy included difficulties in administering medication (31%), insufficient improvement in clinical signs (23%), side effects (16%) and following the referring veterinarian's advice (16%). Almost a fifth of owners (18%) were not informed about the existence of 131I therapy by their veterinarian and found information on 131I treatment online or through friends. Hospitalising their cat was very distressing for 17% of owners. Most owners (92%) were satisfied with the treatment. Reasons for dissatisfaction were insufficient communication, iatrogenic hypothyroidism, persistent hyperthyroidism and comorbidities post-treatment.

CONCLUSIONS AND RELEVANCE

Our study stresses the importance of communication regarding the possible outcome of 131I treatment, the importance of managing underlying comorbidities before treatment and anticipating the stress of owners during their cat's hospitalisation period. The results of this study could help in improving client communication when advising on 131I treatment.

Hyperthyroid cats and their kidneys: a literature review

Hyperthyroidism and chronic kidney disease (CKD) are common diseases of geriatric cats, and often occur concurrently. Thus, a thorough understanding of the influence of thyroid function on renal function is of significant value for all feline practitioners. Among other effects, hyperthyroidism causes protein catabolism and increases renal blood flow and glomerular filtration rate (GFR). These effects render traditional renal markers insensitive for the detection of CKD in cats with uncontrolled hyperthyroidism. Furthermore, the development of iatrogenic hypothyroidism with over treatment of hyperthyroidism can be detrimental to renal function and may negatively affect long-term survival. This review discusses important diagnostic considerations of feline hyperthyroidism, as well as key treatment modalities, with an emphasis on the use of radioiodine and the importance of post treatment monitoring of thyroid and renal parameters. In Australia, a common curative treatment for cats with benign hyperthyroidism (i.e. thyroid hyperplasia or adenoma) is a fixed dose of orally administered radioiodine, regardless of the serum total thyroxine concentration at the time of diagnosis. This review discusses the long term outcomes of this standard of care in comparison with current, relevant research literature from around the world. Finally, this review explores the use of symmetric dimethylarginine (SDMA) in assessing renal function before and after treatment in hyperthyroid cats. SDMA correlates well with GFR and creatinine in non-hyperthyroid cats, but our understanding of its performance in hyperthyroid cats remains in its infancy.

Feline Comorbidities: Balancing hyperthyroidism and concurrent chronic kidney disease

PRACTICAL RELEVANCE

Both hyperthyroidism and chronic kidney disease (CKD) are common long-term conditions in older cats, which might be diagnosed concurrently or develop at different times. Hyperthyroidism may mask the presence of CKD, and vice versa, by various mechanisms that are described in this review. Hyperthyroidism treatment options should be carefully considered when CKD has also been diagnosed.

CLINICAL CHALLENGES

Although it can be difficult to diagnose hyperthyroidism and CKD simultaneously, given that one condition may mask the other, it is important to consider the presence of both diseases when examining an older cat presenting with vomiting, weight loss, polyuria/ polydipsia, anorexia or sarcopenia. The concurrent presence of hyperthyroidism and CKD requires careful monitoring of glomerular filtration rate biomarkers, and adequate and prompt support of kidney function when normal thyroid function is re-established. Iatrogenic hypothyroidism is a recognised complication of all of the treatment options for hyperthyroidism, and increases the risk of azotaemia. Therapy with levothyroxine is recommended for cats that are hypothyroid and azotaemic.

EVIDENCE BASE

The information in this review draws on current literature and guidelines related to the pathophysiology, diagnosis and treatment recommendations for feline hyperthyroidism and CKD.

Evaluation of potential thiamazole exposure of owners of orally treated hyperthyroid cats

OBJECTIVES

The objective of this study was to evaluate the presence of traces of thiamazole in the urine of owners of hyperthyroid cats treated with antithyroid drugs.

METHODS

Urine was collected from 24 owners of hyperthyroid cats, five human patients treated with thiamazole and five healthy humans without any contact with antithyroid drugs. All owners of hyperthyroid cats were asked to fill out a questionnaire. Urine of hyperthyroid cats was collected by spontaneous micturition. All urine samples were stored at -20°C until analysis by ultra-high-performance liquid chromatography coupled to high-resolution quadrupole Orbitrap mass spectrometry.

RESULTS

These owners were assessed to have a lot of contact with their cat. Adherence to antithyroid medication handling guidelines was rather poor. High concentrations of thiamazole were detected in all feline samples (median concentration 2818 ng/ml; range 104-15,127) and in the urine of all human patients treated with thiamazole (median concentration 4153 ng/ml; range 1826-5009). No thiamazole was detected in the urine of owners of hyperthyroid cats (limit of detection 3.88 ng/ml; limit of quantification 11.75 ng/ml).

CONCLUSIONS AND RELEVANCE

The results regarding the potential exposure of owners of hyperthyroid cats to antithyroid drugs are reassuring. Nevertheless, prudence is still warranted when administering antithyroid drugs. Whether these results can be extrapolated to the use of transdermal application requires further investigation.

Utility of a canine TSH assay for diagnosis and monitoring of feline hyperthyroidism

OBJECTIVE

This retrospective study was initiated to evaluate the utility of TSH measurements using a common canine TSH assay to diagnose and monitor feline hyperthyroidism after radioiodine or thyreostatic drug treatment.

MATERIAL AND METHODS

The electronic database of the University of Veterinary Medicine Vienna was searched for combined TSH and T4 measurements. 217 pairs of TSH and T4 from 136 cats with possible hyperthyroidism were assigned to group A (untreated; n = 24) and B (treated; n = 193). Measurements in group B were then subcategorized according to T4 concentrations (reference range 15-50 nmol/L): group B1 = elevated T4 (n = 46), group B2 = normal T4 (n = 84) and group B3 = decreased T4 (n = 63). Group B2 was further divided into cats with low normal (group B2a; n = 35), medium normal (group B2b; n = 29) and high normal (group B2c; n = 20) T4 concentrations.

RESULTS

TSH was detectable in 4 (17 %) of the 24 untreated cats (group A) and did not return to normal despite seemingly successful therapy in two. Increased TSH concentrations were observed in 3.6 % of the treated cats in group B2 and 2.9 %, 6.9 %, and 0 % in subgroups B2a, B2b and B2c, respectively. Forty-four percent of the treated cats with a decreased T4 (group B3) had an increased TSH concentration. TSH correlated with treatment length (r = 0.358, p = 0.004) and was significantly higher in cats treated for more than 3 months (p = 0.008).

CONCLUSION

TSH was detectable in a significant number of untreated hyperthyroid cats and thus this parameter should not be used to definitively rule out feline hyperthyroidism. Furthermore, the very low prevalence of increased TSH concentrations in treated hyperthyroid cats with a normal T4 and cost benefit calculations do not support the routine measurement of TSH in these cats. The fact that TSH correlated with time since treatment start and 56 % of the cats with a decreased T4 had TSH concentrations within the reference limits, suggests delayed recovery of the pituitary thyrotrophs which might explain the low prevalence of subclinical hypothyroidism in the present study.

CLINICAL RELEVANCE

TSH measurement in cats with suspected or treated hyperthyroidism using a canine assay lacks diagnostic sensitivity and can only complement therapeutic decision-making.

Protective Effect of Natural Antioxidant Compounds on Methimazole Induced Oxidative Stress in a Feline Kidney Epithelial Cell Line (CRFK)

The treatment of choice for feline hyperthyroidism is the administration of the antithyroid drug methimazole. Both the endocrinopathy and the drug adverse reactions (e.g., hepatotoxicosis, gastrointestinal disorders, and renal injury) are partly due to oxidative stress and redox unbalance. This study investigated the free radical production and the impairment of the antioxidant barrier induced by methimazole in an in vitro model of feline renal epithelium. The protective effects of quercetin and resveratrol were also explored. CRFK cells were incubated with a methimazole concentration equivalent to the maximum plasma levels in orally treated cats (4 µM), in the presence or absence of either one of the two selected antioxidants at different time-points (up to 72 h). Cell viability, ROS production, GSH levels, and mRNA expression of antioxidant enzymes (i.e., CAT, SOD, GPx, and GST) were assessed. Methimazole impaired cell viability and increased ROS levels in a time-dependent manner. Similarly, GSH content and CAT, SOD, and GPx3 expression were higher compared with control cells. Such effects were significantly counteracted by quercetin. These results provide new insights about the mechanisms underlying the methimazole-related side effects frequently observed in hyperthyroid cats. They also support the use of quercetin in the management of feline hyperthyroidism.

Management of Feline Hyperthyroidism and the Need to Prevent Oxidative Stress: What Can We Learn from Human Research?

Feline hyperthyroidism is a clinical syndrome related to an excessive production of thyroid hormones, and it is considered as a spontaneous animal model for human thyrotoxicosis. Many shared features between the feline and the human disease have been identified so far, including pathogenesis, clinical signs, and treatment options. Although methimazole is considered the first-choice drug in both species, several side effects have been described. Furthermore, methimazole could interfere with the oxidative status, already perturbated by the disease. It has been reported in humans that dietary management, mainly through antioxidant supplementation, could mitigate this oxidative burden. The purpose of the review is to describe current therapeutic options in the course of feline hyperthyroidism and to summarize the state of the art relationship between dietary antioxidants administration and the reduction of methimazole side-effects in humans to support the use of this approach also in cats.

Treatment failure in hyperthyroid cats after radioiodine (I-131) injection

Background

There is limited published information on the outcome for cats where total thyroxine concentration (TT4) remains elevated after treatment with radioactive iodine (RAI).

Objective

To determine the frequency of, and predictors for, subsequent treatment failure in cats for which TT4 remains elevated at hospital discharge, and to report clinical outcomes for cats requiring repeat treatment.

Animals

One hundred twenty‐one cats with TT4 ≥40 nmol/L after treatment with RAI (out of an original, treated study sample of 959 cats).

Methods

Retrospective study. Data regarding signalment, weight, TT4 concentration (before RAI treatment, at discharge, and percentage change), day of sampling, and I‐131 dose were acquired. Logistic regression was performed to evaluate predictors of treatment failure.

Results

In the 87 cats for which classification was possible, 35 (40%) became euthyroid without further treatment. All TT4 variables and weight normalized RAI dose were independently predictive of subsequent treatment failure. In multivariate analysis, TT4 concentration at discharge (P < .001) and weight normalized RAI dose (P = .04) remained in the final model. All 28 cats with TT4 concentration ≥150 nmol/L at discharge ultimately failed treatment, compared with 13/40 (32.5%) and 11/19 (57.9%) cats with TT4 concentrations of 40‐100 nmol/L and 100‐150 nmol/L, respectively. Of the 52 cats that failed treatment, 14 were subsequently managed medically, 12 underwent thyroidectomy (4 with carcinoma), 14 had repeat RAI treatment which was successful in 12/14 (86%) cats, and 13 had no further treatment.

Conclusions and Clinical Importance

Cats with TT4 >150 nmol/L at discharge after RAI might be candidates for immediate repeat treatment.

Assessment of treatment outcomes in hyperthyroid cats treated with an orally administered fixed dose of radioiodine

OBJECTIVES

The aims of this study were to describe the treatment outcomes following oral administration of a fixed dose (138 MBq; 3.7 mCi) of radioiodine in hyperthyroid cats and to examine the correlation between total thyroxine (TT4) concentrations before and after treatment.

METHODS

This was a retrospective cohort study that documented the TT4 concentration and clinicopathological parameters at the time of diagnosis and after treatment. Logistic regression was used to assess the relationship between TT4 concentrations before and after treatment. The difference in pre- and post-treatment variables between cats that had TT4 concentrations below or within the reference interval (RI) was compared by the Mann-Whitney U-test.

RESULTS

Of 161 cats, 133 (82.6%) cats had TT4 concentrations within the RI, four (2.5%) cats had TT4 concentrations above the RI and 24 (14.9%) cats had TT4 concentrations below the RI after treatment. The severity of hyperthyroidism at diagnosis, as measured by the percentage of TT4 elevation above the upper limit of the RI, had no impact on the odds of cats having low TT4 concentrations after treatment (odds ratio 1.00; 95% confidence interval 0.96-1.05; P = 0.828).

CONCLUSIONS AND RELEVANCE

When using an orally administered fixed dose of radioiodine for the treatment of feline hyperthyroidism, TT4 concentrations at diagnosis cannot be used to predict TT4 concentrations after treatment. The proportion of cats with TT4 concentrations below the lower limit of the RI after treatment was 14.9%. Further work is required to optimise oral radioiodine dosing to achieve maximal euthyroid outcomes.

Hyperthyroidism in Cats: Considering the Impact of Treatment Modality on Quality of Life for Cats and Their Owners

In cats, hyperthyroidism can be treated in 4 ways: medical management with methimazole or carbimazole, nutritional management (low-iodine diet), surgical thyroidectomy, and radioactive iodine (¹³¹I). Each form of treatment has advantages and disadvantages that should be considered when formulating a treatment plan for the individual hyperthyroid cat. Medical and nutritional managements are considered "reversible" or palliative treatments, whereas surgical thyroidectomy and ¹³¹I are "permanent" or curative treatments. The author discusses how each treatment modality could be the optimal choice for a specific cat-owner combination and reviews the advantages and disadvantages of each treatment option.

Hyperthyroidism is not a risk factor for subclinical bacteriuria in cats: A prospective cohort study

Background

Subclinical bacteriuria is defined as a positive bacterial urine culture in the absence of clinical evidence of urinary tract infection (UTI). Studies have reported that hyperthyroid cats have UTIs (mostly subclinical) with prevalence rates of 12%‐22%. Consequently, clinicians consider hyperthyroidism a risk factor for development of subclinical bacteriuria, and many recommend urine culture when evaluating hyperthyroid cats.

Objectives

To compare the prevalence of subclinical bacteriuria (based on positive urine culture) in untreated hyperthyroid cats to that in euthyroid cats of similar age and sex.

Animals

Three hundred and ninety‐three hyperthyroid cats presented for radioiodine treatment and 131 euthyroid cats (≥7 years of age) presented for routine examination. Cats with signs of lower urinary tract disease were excluded.

Methods

Prospective cohort study. Both hyperthyroid and euthyroid cats had urine collected by cystocentesis for complete urinalysis and culture. Data pertaining to age, sex, body condition, and serum thyroxine and creatinine concentrations also were acquired. Logistic regression was performed to evaluate for potential risk factors for subclinical bacteriuria.

Results

Hyperthyroid cats showed a low prevalence of subclinical bacteriuria (4.3%), which did not differ from that found in euthyroid cats (4.6%). Of the signalment factors evaluated, only female sex was a significant risk factor (odds ratio [OR], 6.9; P = .002). Furthermore, positive urine cultures were more likely in specimens with dilute urine concentration (<1.035), pyuria, or microscopic bacteriuria.

Conclusions and Clinical Importance

Hyperthyroid cats are not at risk for subclinical bacteriuria. In the absence of lower urinary tract signs, no clinical benefit exists in routinely performing urine cultures when evaluating hyperthyroid cats.

Evaluation of Antioxidant Supplementation on Redox Unbalance in Hyperthyroid Cats Treated with Methimazole: A Blinded Randomized Controlled Trial

Methimazole (MMI) is often the selected medical treatment for feline hyperthyroidism. However, the onset of MMI-related side effects (MMI-SE) is likely caused by oxidative stress. This study evaluated the dietary supplementation of selected antioxidants in hyperthyroid cats receiving MMI, to reduce MMI-SE. Thirty hyperthyroid client-owned cats were randomly allocated in group M (MMI + placebo) or group M+A (MMI + antioxidants). At different time-points from the enrolment (ET) to the end of the trial (FT), the following information was recorded: clinical findings, complete blood count, serum biochemical parameters, urinalysis, total plasma thyroxine concentrations, determinable reactive oxygen metabolites (dROMs), OXY-adsorbent test values, and oxidative stress index (OSi) values, and MMI-SE. dROMs and OSi values significantly increased from ET to FT in group M and were significantly higher in group M than in group M+A at FT. Likewise, OXY-absorbent test values were significantly higher in group M+A than in group M at FT. Moreover, the occurrence rate of MMI-SE in group M+A was lower than in group M. In conclusion, our results show that the dietary supplementation of antioxidants in hyperthyroid cats receiving MMI exerts a protective effect against oxidative stress, likely contributing to the reduction of MMI-SE.

Prevalence of iatrogenic hypothyroidism in hyperthyroid cats treated with radioiodine using an individualised scoring system

OBJECTIVES

The aim of this study was to report the prevalence of iatrogenic hypothyroidism, with or without azotaemia, based on the measurement of serum total thyroxine (T4), thyroid-stimulating hormone (TSH) and creatinine concentrations, in hyperthyroid cats undergoing radioiodine (131I) treatment where the 131I dose was calculated using a previously described scoring system. A secondary aim of the study was to determine the positive and negative predictive values of serum T4 and TSH concentrations obtained 19 days after treatment in order to predict the development of iatrogenic hypothyroidism 6-9 months after 131I treatment.

METHODS

Serum T4, TSH and creatinine concentrations were measured 19 days and 6-9 months after 131I treatment. The prevalence of iatrogenic hypothyroidism was assessed with the results obtained 6-9 months after 131I treatment.

RESULTS

The prevalence of overt and subclinical hypothyroidism 6-9 months after 131I treatment was 40.0% (22/55 cats) and 12.7% (7/55 cats). Overt hypothyroidism with azotaemia was diagnosed in 8/55 (14.5%) cats. The positive and negative predictive values for the prediction of the development of iatrogenic hypothyroidism 6-9 months after 131I treatment were 72.2% and 80.0%, respectively, for a low serum T4 concentration, and 75.0% and 44.6%, respectively, for an increased serum TSH concentration.

CONCLUSIONS AND RELEVANCE

The use of an individualised scoring system is effective in determining the 131I dose for the treatment of hyperthyroid cats. However, the prevalence of overt hypothyroidism was higher in comparison with other studies using different dosing protocols. Further studies comparing the efficacy of individualised scoring systems and different fixed doses to determine which method is superior are warranted.

Iodine-restricted food versus pharmacological therapy in the management of feline hyperthyroidism: A controlled trial in 34 cats

Background:

Hyperthyroidism is a common endocrinopathy of middle-aged and elderly cats. Dietary treatment has been proposed as an alternative to traditional therapies.

Aim:

The aim of this prospective study was to compare the efficacy of iodine-restricted food versus pharmacological therapy with methimazole in client-owned cats with hyperthyroidism.

Methods:

Indoor cats with newly diagnosed hyperthyroidism (consistent clinical signs and serum total thyroxine concentration greater than 50 nmol/l) were assigned to one of three groups: (A) received an iodine-restricted food as a single therapy; (B) received transdermal methimazole in pluronic lecithin organogel; and (C) received oral methimazole. In all groups, clinical parameters, biochemistry, and serum total thyroxine were evaluated at baseline and 10, 30, 60, and 90 days after treatment began.

Results:

Thirty-four cats were enrolled in the study (group A: n = 14; group B: n = 11; group C: n = 9). No significant differences were found between groups at diagnosis for signalment, clinical and laboratory findings, including serum total thyroxine concentrations. In all the groups, serum total thyroxine concentration decreased significantly following the baseline measurement. After 90 days of treatment, serum creatinine increased significantly only in the methimazole-treated groups. Liver enzyme activities decreased significantly only in group B, while no significant decreases were detected in groups A and C at any time.

Conclusion:

These results suggest that iodine-restricted food is effective at reducing the total thyroxine concentration in the serum of hyperthyroid cats. Moreover, the iodine-restricted food did not cause any increase in serum creatinine concentrations and failed to improve liver enzymes abnormalities. These findings could indicate a persistent hyperthyroid state in cats treated with iodine-restricted food despite normalization of serum total thyroxine concentrations.

Redox unbalance in the hyperthyroid cat: a comparison with healthy and non-thyroidal diseased cats

Background

Feline hyperthyroidism, the most common endocrinopathy in older cats, provides a spontaneous model for human thyrotoxicosis. Human thyrotoxicosis is associated with redox unbalance, which may result in organ damage. The redox status of hyperthyroid cats is largely unknown. The aims of the present study were to compare the redox status of cats with hyperthyroidism with that of healthy cats and cats with chronic non-thyroidal illness.

Results

Forty cats with untreated hyperthyroidism (group H), 45 chronically ill cats with non-thyroidal illness (group I), and 39 healthy cats (group C) were recruited for this observational cross-sectional study. All cats were screened for redox status markers. Determinable reactive oxygen metabolites (d-ROMs) were used as oxidative stress markers. Antioxidant status was determined using the OXY-Adsorbent test to quantify the plasma barrier to oxidation. The Oxidative Stress index (OSi) was calculated as the ratio of d-ROMs and OXY-Adsorbent test values. Data were compared by ANOVA with Tukey’s multiple comparisons post-hoc test. The dROMs of group H (193 ± 47 CarrU) were significantly higher (p &lt; 0.001) than those of the healthy cats (103 ± 17 CarrU). The OXY-Adsorbent test results in group H (265 ± 68 μmol HClO/ml) were significantly lower than those in healthy cats (390 ± 83 μmol HClO/ml; p &lt; 0.01) and chronically ill cats (306 ± 45 μmol HClO/ml, p &lt; 0.05). Moreover, the Osi value in group H (0.8 ± 0.2 CarrU/μmol HClO/ml) was significantly higher (p &lt; 0.001) than that of the healthy cats (0.3 ± 0.1 CarrU/μmol HClO/ml).

Conclusions

As described in humans with hyperthyroidism, feline hyperthyroidism is associated with redox unbalance. Free radical production is increased in hyperthyroid cats and their antioxidant depletion seems to be more severe than in cats with non-thyroidal illnesses. Our results support the rationale for a clinical trial investigating the potential positive effects of antioxidant supplementation to cats with hyperthyroidism.

Electronic supplementary material

The online version of this article (10.1186/s12917-019-1896-7) contains supplementary material, which is available to authorized users.

Expected total thyroxine (TT4) concentrations and outlier values in 531,765 cats in the United States (2014-2015)

Background

Levels exceeding the standard reference interval (RI) for total thyroxine (TT4) concentrations are diagnostic for hyperthyroidism, however some hyperthyroid cats have TT4 values within the RI. Determining outlier TT4 concentrations should aid practitioners in identification of hyperthyroidism. The objective of this study was to determine the expected distribution of TT4 concentration using a large population of cats (531,765) of unknown health status to identify unexpected TT4 concentrations (outlier), and determine whether this concentration changes with age.

Methodology/Principle findings

This study is a population-based, retrospective study evaluating an electronic database of laboratory results to identify unique TT4 measurement between January 2014 and July 2015. An expected distribution of TT4 concentrations was determined using a large population of cats (531,765) of unknown health status, and this in turn was used to identify unexpected TT4 concentrations (outlier) and determine whether this concentration changes with age. All cats between the age of 1 and 9 years (n = 141,294) had the same expected distribution of TT4 concentration (0.5–3.5ug/dL), and cats with a TT4 value >3.5ug/dL were determined to be unexpected outliers. There was a steep and progressive rise in both the total number and percentage of statistical outliers in the feline population as a function of age. The greatest acceleration in the percentage of outliers occurred between the age of 7 and 14 years, which was up to 4.6 times the rate seen between the age of 3 and 7 years.

Conclusions

TT4 concentrations >3.5ug/dL represent outliers from the expected distribution of TT4 concentration. Furthermore, age has a strong influence on the proportion of cats. These findings suggest that patients with TT4 concentrations >3.5ug/dL should be more closely evaluated for hyperthyroidism, particularly between the ages of 7 and 14 years. This finding may aid clinicians in earlier identification of hyperthyroidism in at-risk patients.

Changes in thyroid and renal function after bilateral thyroidectomy in cats

BACKGROUND

Iatrogenic hypothyroidism might worsen the prognosis of cats with azotemic CKD after thyroidectomy. Varying thyroxine concentrations influence utility of creatinine in assessing renal function. Symmetric dimethylarginine (SDMA) has limited studies in cats with changing thyroid status.

OBJECTIVES

Thyroid status is stable 6 months post-thyroidectomy. Symmetric dimethylarginine and creatinine are linearly associated without influence from total thyroxine concentration (tT4).

ANIMALS

Electronic records of 2 first opinion practices were searched using the term "thyroidectomy" to include 81 client-owned cats that had undergone bilateral thyroidectomy.

METHODS

Retrospective cross-sectional study assessing thyroid hormone concentrations of 68 cats within 6 months of surgery. A longitudinal study of thyroid status in 23 cats with >18 months follow-up post-thyroidectomy. A generalized estimating equation assessed the associations of bodyweight, tT4 and creatinine concentrations on SDMA concentration.

RESULTS

Sixty-eight cats had follow-up within 6 months. Fifteen cats (22%) had persistent, or recurrent, hyperthyroidism and 33 cats (49%) were hypothyroid. Twenty-three of the euthyroid/hypothyroid cats had long-term follow-up (595-1955 days); 4 cats (17%) remained hypothyroid, 19 cats (83%) were euthyroid (often transiently), and 9 of 23 cats (44%) developed recurrent hyperthyroidism. Symmetric dimethylarginine and creatinine were linearly associated, but hyperthyroid cats had higher SDMA concentrations, relative to creatinine (P = .003).

CONCLUSIONS AND CLINICAL IMPORTANCE

Cats have changes in thyroid function for years after bilateral thyroidectomy, with a high incidence of recurrent hyperthyroidism. Both SDMA and creatinine are affected by thyroxine concentrations, and the effect is greater in hyperthyroid cats.

Management and monitoring of hyperthyroid cats: a survey of Australian veterinarians

Objectives This study sought to evaluate how Australian veterinarians approach management and monitoring of feline hyperthyroidism and compare these results with a similar survey recently performed in the UK. Methods An invitation to complete an online survey was sent to veterinarians in all states and territories of Australia. The survey comprised questions relating to management of hyperthyroidism, use of antithyroid drugs vs radioiodine treatment vs surgical thyroidectomy, in addition to demographic information for respondents. Results A total of 546 clinicians completed the survey. The most commonly preferred treatments for long-term management of feline hyperthyroidism were antithyroid medications (305/546; 56%) and radioiodine (210/546; 38%), with substantially more respondents selecting radioiodine when cost was removed as a consideration (425/546; 78%). However, most respondents had treated or referred few cases for radioiodine (median 2). Most veterinarians (500/546; 92%) used antithyroid medications either long term or prior to definitive treatment of hyperthyroidism. For medical management, 45% (244/546) of veterinarians used twice-daily carbimazole. Half of respondents (274/546) aimed to maintain the total thyroxine concentration anywhere within the laboratory reference interval in hyperthyroid cats without chronic kidney disease. Blood pressure monitoring was uncommon. Surgical thyroidectomy was rarely performed. Conclusions and relevance Radioiodine was more frequently preferred by Australian veterinarians compared with those in the UK, likely associated with greater availability, reduced cost and shorter hospitalisation times in this jurisdiction, although antithyroid medications were the most frequently used treatment modality. Barriers remain to its utilisation, however, including perceived cost, misconceptions with regard to expected success rate and accessibility. Recent changes to recommendations on the management and monitoring of hyperthyroid cats do not appear to have been widely adopted by veterinarians at this time.

Life-threatening haematological complication occurring in a cat after chronic carbimazole administration

Case summary

An 11-year-old spayed female domestic shorthair cat with a history of hyperthyroidism treated with carbimazole for 7 months was presented for a check-up after a few episodes of vomiting. The cat had been receiving prednisolone at 0.5 mg/kg PO q12h for recent pancreatitis and concurrent inflammation of liver and small intestines confirmed by biopsies. Clinical examination revealed pale mucous membranes with a capillary refill time of <2 s. Haematology showed severely decreased packed cell volume (16%), and increased prothrombin time (42 s), partial thromboplastin time (>120 s) and fibrinogen serum concentration (3.5 g/l). Morphological changes of thrombocytes in the absence of thrombocytopenia were also noted. In-saline agglutination test was positive. Abdominal radiographic and ultrasonographic examinations excluded the presence of organ abnormalities and peritoneal effusion. Blood biochemistry was unremarkable. Feline leukaemia virus and feline immunodeficiency virus tests were negative. On the basis of these findings, immune-mediated anaemia secondary to chronic carbimazole administration was suspected. Prednisolone was increased to 2 mg/kg PO q24h and carbimazole tablets were stopped. Despite close monitoring and intensive care, the cat died the same evening of admission to the hospital.

Relevance and novel information

This report suggests that severe haemotoxicity may occur as a sequel of chronic carbimazole administration in cats. Routine bloodwork and accurate follow-up of cats under treatment with thyrotoxic therapy may be advisable, in order to detect haematological changes before lethal complications occur.

Efficacy of Low-dose (2 millicurie) versus Standard-dose (4 millicurie) Radioiodine Treatment for Cats with Mild-to-Moderate Hyperthyroidism

Background

Radioiodine (¹³¹I) is effective treatment for hyperthyroidism in cats, but optimal dose to restore euthyroidism without inducing hypothyroidism is unclear. Treatment‐induced hypothyroidism can lead to azotemia and reduced duration of survival.

Objective

To compare efficacy and short‐term outcomes of low‐dose ¹³¹I versus higher, standard‐dose ¹³¹I as treatment for hyperthyroidism.

Animals

A total of 189 client‐owned cats undergoing ¹³¹I treatment for mild‐to‐moderate hyperthyroidism (serum T₄ ≥ 4.0 μg/dL and <13.0 μg/dL).

Methods

Prospective, nonrandomized, cohort study comparing treatment with either low‐dose (2 mCi, n = 150) or standard‐dose (4 mCi, n = 39) ¹³¹I. Serum T₄, thyroid‐stimulating hormone (TSH), and creatinine concentrations were measured after 1, 3, and 6 months to determine persistent hyperthyroidism, overt hypothyroidism (low T₄, high TSH), subclinical hypothyroidism (normal T₄, high TSH), and azotemia.

Results

There was no significant difference in prevalence of cats with persistent hyperthyroidism between standard‐ and low‐dose treatment groups at 3 (0% versus 5.3%; P = .34) and 6 (0% versus 3.3%; P = .51) months. Overt (18% versus 1%; P = .0005) or subclinical (46% versus 21%; P = .004) hypothyroidism was more common in cats at 6 months after standard‐dose ¹³¹I. No difference in incidence of azotemia existed between groups, but cats treated with standard‐dose ¹³¹I had higher creatinine concentrations (P < .05) and higher percent rises in creatinine (P < .0001).

Conclusions and Clinical Importance

Low‐dose ¹³¹I is safe and effective for cats with mild‐to‐moderate hyperthyroidism, as evidenced by a cure rate of >95% with reduced frequency of iatrogenic hypothyroidism and azotemia.

Rodent Oncology: Diseases, Diagnostics, and Therapeutics

Cancer incidence in rodent species varies dramatically from a common occurrence in mice and rats to just a limited number of documented cases in chinchillas and degus. This article summarizes common tumors, both benign and malignant, that have been reported to occur in rodents. Outlined are clinical signs, diagnostics, and treatments that have been described for rodents presenting with specific neoplasms.

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.

Effect of Feeding an Iodine-Restricted Diet in Cats with Spontaneous Hyperthyroidism

BACKGROUND

Exclusive feeding of an iodine-restricted diet has been proposed as a method for controlling clinical manifestations of hyperthyroidism in hyperthyroid cats.

OBJECTIVES

To determine the effect of feeding an iodine-restricted diet on TT4 concentrations and clinical signs in cats with spontaneous hyperthyroidism.

ANIMALS

Forty-nine client-owned cats with spontaneous hyperthyroidism.

METHODS

Retrospective case series. Hyperthyroid cats were exclusively fed a commercially available iodine-restricted diet. Clinical response was assessed by change in weight and heart rate and serum TT4, blood urea nitrogen (BUN), and creatinine concentrations at various times during dietary management (21-60 days, 60-180 days).

RESULTS

Serum TT4 normalized in 20/48 cats (42%) and 39/47 cats (83%) at 21-60 days and 61-180 days, respectively. Cats in which the TT4 concentrations were still above reference range at 21-60 days had a significantly higher starting TT4 than those that normalized their TT4 levels during the same time period (P = .038). Body weight did not significantly increase (P = .34) nor heart rate decrease (P = .64) during the study. There was a significant decrease in serum creatinine (P = .028). Cats in the low reference range for serum TT4 concentrations did not have a significant increase in body weight (P = .41) nor creatinine (P = .54) when compared to those with high reference range.

CONCLUSIONS AND CLINICAL IMPORTANCE

Restricted-iodine diets were effective at maintaining serum TT4 concentrations within reference ranges for a majority of cats with spontaneous hyperthyroidism over 1 year, although not all clinical signs of hyperthyroidism improved.

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.

Diagnosis and management of feline hyperthyroidism: current perspectives

Previous and ongoing research has provided insights to the pathophysiology and diagnosis of hyperthyroidism as well as new treatment modalities. This paper reviews the etiology, clinical presentation, and clinicopathologic changes associated with hyperthyroidism, and provides a thorough explanation of confirmatory testing and treatment options.