Effects of an iodine-restricted food on client-owned cats with hyperthyroidism

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.

A systematic review of the quality of life assessment tools for cats in the published literature

Quality of life (QoL) is an important parameter to assess in cats, as it can be pivotal to important decision-making. Research reports that owners of cats with heart disease would trade longevity for QoL, and treatment associated improvement in QoL is very important for cats with chronic kidney disease. This systematic review aimed to explore the published literature to identify the number and range of QoL assessment tools available to researchers and veterinary professionals, by discovering tools which have already been used in published studies. Medline and CAB Abstracts were searched in March 2018, using terms relevant to cats and QoL or well-being. Inclusion and exclusion criteria were applied and information on uniqueness, validation and a short description of each tool extracted. A total of 1138 manuscripts were identified, of which 96 met all criteria. Forty of 96 manuscripts contained an assessment of QoL, using one of 32 unique tools identified. Sixteen of the tools identified were structured, making detailed patient assessments. Only eight of the structured tools were validated, and of these, three could be applied to healthy cats; the remainder being specific to a disease or being hospitalised. Some validated tools appeared in more than one manuscript. Overall, 12 manuscripts used a validated tool. In the 16 unstructured tools, five tools assessed QoL by assigning a single word (e.g. 'poor'). Eight tools assessed QoL on a single Likert scale (e.g. a number between 1 and 5). This work identifies the tools that are currently available for the assessment of QoL by researchers and veterinary professionals. Additionally, it demonstrates that many are not validated or lack detailed animal assessment, highlighting that further work in this important area is needed.

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.

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.

One-year study evaluating efficacy of an iodine-restricted diet for the treatment of moderate-to-severe hyperthyroidism in cats

PURPOSE

This study sought to determine the efficacy of a therapeutic iodine-restricted diet during a 12 -month interval in cats with moderately to severely increased total thyroxine (TT4) concentrations.

PATIENTS AND METHODS

Eight hyperthyroid cats with serum creatinine <2.0 mg/dL and TT4 >6.0 µg/dL (reference interval 1.5-4.0 µg/dL) were enrolled. Each cat underwent an initial physical examination, complete blood count, serum chemistry panel, thyroid profile, thyroid scintigraphy, and echocardiogram. Physical examination and all blood samplings were repeated at 1, 2, 3, 6, and 12 months after enrollment. Clients were instructed to feed only the therapeutic iodine-restricted diet throughout the entire study.

RESULTS

Median TT4 was 8.4 (range 6.2-24.0) µg/dL at study onset. Thyroid scintigraphy was abnormal in all cats, confirming hyperthyroidism. Six out of eight cats had normal serum TT4 after 4 weeks of feeding the therapeutic diet. The two cats that did not achieve normalization of TT4 at 4 weeks had the highest initial TT4 concentrations. Three cats were withdrawn from the study due to emergence of chronic kidney disease.

CONCLUSION

An iodine-restricted diet was able to control moderate-to-severe hyperthyroidism in most (six out of eight) of the cats by 4 weeks. Cats with highest baseline TT4 took substantially longer before TT4 concentrations normalized, if at all.

Survival times for cats with hyperthyroidism treated with a 3.35 mCi iodine-131 dose: a retrospective study of 96 cases

Objectives Radioiodine (131I) dose determination using radiotracer kinetic studies or scoring systems, and fixed relatively high 131I dose (ie, 4 or 5 mCi) administration, are effective and associated with prolonged survival times for hyperthyroid cats. The latter method is less complicated but could expose patients and veterinary personnel to unnecessary levels of radiation. The aim of this study was to retrospectively evaluate the efficacy of a fixed 3.35 mCi 131I dose for the treatment of 96 hyperthyroid cats with no length estimation for any palpated goitre ⩾20 mm, assess outcome and identify factors associated with survival. Methods Serum total thyroxine concentrations at diagnosis and at follow-up times, survival times and cause of death were recorded. Multivariable Cox regression analysis was used to identify factors associated with time to any cause of death from 131I therapy initiation. Results Administration of a median (interquartile range) dose of 3.35 mCi (3.27-3.44 mCi) radioiodine was an effective treatment in 94/96 cats, but two cats remained hyperthyroid. No death related to hyperthyroidism was recorded. Median survival time was 3.0 years; the 1 and 2 year survival rates after 131I therapy were 90% and 78%, respectively. Low body weight (⩽3.1 kg; adjusted hazard ratio [aHR] 5.88; 95% confidence interval [CI] 2.22-16.67; P <0.01) and male gender (aHR 2.63; 95% CI 1.01-7.14; P = 0.04) were independently associated with death, whereas age, prior treatment with antithyroid drugs, reason for treatment and pretreatment azotaemia were not. Conclusions and relevance This study suggests that a fixed 3.35 mCi 131I dose treatment is effective for hyperthyroid cats with goitre(s) with a maximal length estimation <20 mm, that long-term survival can be achieved and that low body weight and male gender are significantly associated with shorter survival times.

Comparison of health parameters in normal cats fed a limited iodine prescription food vs a conventional diet

Objectives The objective was to compare the effect on thyroid function in healthy, adult cats fed a limited-iodine food or conventional diet for 24 months. Methods Cats in the limited-iodine group (n = 14) were fed a commercial, dry food containing 0.2 ppm iodine on a dry-matter basis (DMB). Cats in the conventional diet group (n = 12) were fed an identical diet except that the iodine content was 3.2 ppm on a DMB. Both groups were maintained on their respective diets for 24 months. The median age of the cats at baseline was 3.2 years (interquartile range 2.4-4.3). Diagnostic samples were obtained for measurement of serum biochemistry parameters, thyroid hormone concentrations, complete blood count and urinalysis, and thyroid ultrasound examination was performed at baseline, 6, 12, 18 and 24 months. Results Median serum concentrations of free and total thyroxine and thyroid-stimulating hormone were within respective reference intervals for both test groups at all sampling intervals. Median urine iodine concentrations in the limited-iodine group declined significantly ( P = 0.0001) from baseline and were significantly different than conventional diet ( P ⩽0.0007). Ultrasound examination revealed no significant change in median thyroid height in the limited-iodine group at any time point. Conclusions and relevance Normal serum thyroid hormone concentrations and results of urinalysis and routine biochemical testing confirmed that the limited-iodine food was safely fed for 2 years to healthy, adult cats. Ultrasound examination showing that cats on limited-iodine did not develop a change in thyroid gland height was a further indication of the absence of iatrogenic hypothyroidism in the limited iodine diet group. These results support the lack of obvious side effects associated with feeding a limited-iodine diet for 2 years to healthy, adult cats, a situation that may occur in multi-cat households where healthy cats have access to a limited-iodine diet being used to manage cats with feline hyperthyroidism.

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.

Double-blinded randomised placebo-controlled clinical trial of individualised homeopathic treatment of hyperthyroid cats

Feline hyperthyroidism is a common endocrine disorder in older cats for which homeopathic treatment has been advocated. A double-blinded, placebo-controlled randomised trial was performed to look for evidence of efficacy for the use of individualised homeopathy in the treatment of this disease. Using a case definition of a concentration of the thyroid hormone T4 >66 nmol/l, cats were randomised into two treatment arms. Either a placebo or a homeopathic treatment was given to each cat blindly. After 21 days, the T4 levels, weight (Wt) and heart rate (HR) were compared with pretreatment values. There were no statistically significant differences in the changes seen between the two treatment arms following placebo or homeopathic treatment (T4 P=0.96, Wt P=0.16, HR P=0.36) or between the means of each parameter for either treatment arm before and after placebo or homeopathic treatment (all P values >0.13). In a second phase of the study, patients in both treatment arms were given methimazole treatment for 21 days and T4, Wt and HR determined again. Again there were no statistically significant differences between the groups, but there were statistically significant reductions in T4 (P<0.0001) and HR (P=0.02), and a statistically significant increase in Wt (P=0.004) in both groups compared with their pre-methimazole treatment levels. The results of this study failed to provide any evidence of the efficacy of homeopathic treatment of feline hyperthyroidism.

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 a Limited Iodine Diet on Iodine Uptake by Thyroid Glands in Hyperthyroid Cats

BACKGROUND

The effect of feeding a limited iodine diet on radioactive iodine uptake in the thyroid glands of hyperthyroid cats is unknown.

OBJECTIVES

To determine how feeding limited dietary iodine affects radioactive iodine uptake by the thyroid glands of hyperthyroid cats.

ANIMALS

Eight geriatric cats with spontaneous hyperthyroidism.

METHODS

Prospective study of eight client owned hyperthyroid cats fed a commercially available iodine limited diet for 6 months. Clinical signs were evaluated and TT4 and fT4 were measured during consumption of the diet. Uptake of (123)I was determined before and 8-16 weeks after exclusive consumption of the diet.

RESULTS

Clinical signs of hyperthyroidism resolved in all cats, but there was no significant increase in body weight. TT4 and fT4 decreased into the reference range by 8-16 weeks in all cats. Mean TT4 before consumption of the diet was 9.7 μg/dL (SD 5.2) and after consumption of the diet was 3.1 μg/dL (SD 0.9). Scintigraphy revealed unilateral uptake of isotope in 5 cats and bilateral uptake in 3 cats. Mean percentage uptake of (123)I by the thyroid gland at 8 hours after isotope administration was 16.2 (SD 11.8) before diet consumption and 34.6 (SD 11.7) 8-16 weeks after exclusive consumption of the diet. The percentage increase was variable between cats (38-639%).

CONCLUSIONS AND CLINICAL IMPORTANCE

Limited iodine diets increase iodine uptake in the autonomous thyroid glands of hyperthyroid cats. Further studies are necessary to determine if consumption of a limited iodine diet changes sensitivity of the thyroid gland to (131)I treatment.

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.

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.