Juvenile hyperthyroidism in a cat

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.

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.

Juvenile hyperthyroidism in a dog

Hyperthyroidism is the most common endocrinopathy in cats, and most cases are caused by multinodular hyperplasia or follicular cell adenoma, although thyroid carcinomas occur infrequently. Hyperthyroidism is rare in dogs, and most cases are caused by functional thyroid carcinomas. There are case reports of canine hyperthyroidism secondary to exogenous sources, and rarely thyrotoxicosis can be seen with therapeutic doses of levothyroxine prescribed for hypothyroidism. A case of juvenile hyperthyroidism has been reported in a cat and a histopathological diagnosis of diffuse thyroid hyperplasia was made. This is the first reported case of functional eutopic thyroid tissue in a young dog. Histopathological examination reported bilateral hyperplastic change in the thyroid glands, similar to the case of feline juvenile hyperthyroidism.

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.

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.

Hyperthyroidism and hyperparathyroidism in guinea pigs (Cavia porcellus)

Abnormal conditions of the thyroid and parathyroid in the guinea pig appear in the English-language scientific literature on an emerging basis. Although true descriptions of abnormal thyroid and parathyroid anatomy or morphology are not new findings, the clinical condition of abnormal thyroid or parathyroid function seems to be a more common observation by clinicians in recent years. This article is an overview of general clinical conditions and adequate diagnosis, and offers treatment options.

Testing for Hyperthyroidism in Cats

Hyperthyroidism remains a common endocrine disorder of cats. Although relatively easy to diagnose in classically presenting cats, the increased frequency of testing cats with early or mild disease has had significant implications for the diagnostic performance of many of the routine tests currently used. Further advances in the etiopathogenesis and earlier diagnosis are only likely with the advent of a species specific feline thyroid-stimulating hormone assay.

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.