Hypothyroidism and hyperthyroidism are commonly-seen disorders in canine and feline patients. Accurate diagnostic testing and screening are vital to successful treatment and management.
Like humans, dogs and cats can be affected by diseases of the thyroid. This article provides a comprehensive discussion of the diagnosis, treatment and management of these frequently-seen disorders.
THYROID DISEASES AND THEIR CAUSES
Hypothyroidism is the most common endocrine disorder in dogs, and the second most common in older cats, after diabetes. At least 80% of canine cases result from heritable autoimmune(lymphocytic) thyroiditis. Because the heritable nature of this disorder poses significant genetic and health implications for breeding stock, accurate diagnosis of the early compensatory stages of canine autoimmune thyroiditis that lead up to clinical hypothyroidism affords important genetic and clinical options for prompt intervention and case management. In cats, hyperthyroidism is readily induced, especially in geriatrics, by feeding commercial pet foods, treats and snacks containing excessive amounts of iodine. Since this discovery, major changes have been made to the iodine content of commercial cat foods.
DIETARY-INDUCED HYPERTHYROIDISM IN DOGS AND CATS
Studies have documented iatrogenic hyperthyroidism in pets fed the gullet or throat portion of raw red meat (usually beef or lamb), if it contains the thyroid gland. The patient may be relatively asymptomatic, with the diagnosis made upon finding significantly high thyroid basal levels without clinical evidence of a thyroid mass in the neck or chest. Questioning the pet owner then reveals the raw meat diet (clients may not be aware that the gullet or throat portion of the carcass is included in the product). Removing the meat from the diet resolves the hyperthyroidism in about four to six weeks, at which time retesting the thyroid levels should show they have returned to normal.
IODINE CONTENT IN COMMERCIAL PET FOODS
An excess of iodine causes alterations in thyroid activity, blocking both its characteristic functions and cell proliferation. Feeding excessive amounts of iodine in foods and supplements(kelp, seaweed) reduces thyroid function in dogs and increases thyroid activity in older cats.
BASELINE CANINE THYROID PROFILES
A complete baseline thyroid profile is measured in dogs, and typically includes total T4, total T3, free T4, freeT3, and TgAA (thyroglobulin autoantibody). It can also include circulating T3 and T4 autoantibodies (T3AAand T4AA), and the canine-specific thyroid stimulating hormone (cTSH). The TgAA assay is an especially important component for screening breeding stock for heritable autoimmune thyroid disease.
GENETIC SCREENING AND DIAGNOSTIC TESTING FOR CANINE THYROID DISEASE
Nearly all cases of canine thyroiditis have elevated serum TgAA levels, whereas only about 20% to 40% of cases have elevated circulating T3AA and/or T4AA. Thus, while the presence of elevated T3AA and/or T4AA also confirms a diagnosis of autoimmune thyroiditis, it underestimates the prevalence, since a negative(non-elevated) level of these circulating autoantibodies does not rule out the presence of thyroiditis. Measuring TgAAlevels also permits early recognition of the disorder, and facilitates genetic counselling since affected dogs should not be used for breeding.
Cumulative data show that females are significantly more likely to have thyroid autoantibodies than males. Thyroid testing for genetic screening purposes is less likely to be meaningful before puberty. Screening is initiated, therefore, once healthy dogs and bitches have reached sexual maturity (between ten and 14 months in males and during the first anestrous period for females following their maiden heat — i.e. at 12to 16 weeks from its onset). As the female sexual cycle is quiescent during anestrus, any influence of sex hormones on baseline thyroid function should be minimized. Canine autoimmune thyroid disease has been documented to be similar to Hashimoto’s lymphocytic thyroiditis in humans. These diseases are associated with a handful of genes from the major histocompatibility complex (MHC) tissue types in people, and in more than a dozen dog breeds and hybrid breeds. Several detailed publications have studied the parallel genes that convey predisposition to, and increased risk for, heritable thyroiditis in dogs.
OTHER ASSOCIATED DISORDERS
POLYGLANDULAR AUTOIMMUNITY
Human and canine individuals who are genetically susceptible to autoimmune thyroid disease may also become more susceptible to immune-mediated diseases affecting other target tissues and organs. Multiple endocrine glands and non-endocrine systems become involved in a systemic immune-mediated process. The most common associated disorders are Addison’s disease, diabetes and reproductive failure. Common associated non-endocrinologic auto-immune disorders include autoimmune/immune-mediated hemolytic anemia (AIHA/IMHA), idiopathic thrombocytopenic purpura (ITP), chronic active hepatitis, and immune-complex glomerulonephritis (systemic lupus erythematosus; SLE).
ABERRANT BEHAVIOR AND THYROID DYSFUNCTION
The principal reason for pet euthanasia stems not from disease, but undesirable behavior, which can reflect underlying problems of a psychological nature. An association between behavioral and psychological changes and thyroid dysfunction has been recognized in humans since the 19th century. A parallel association has been established between aberrant behavior and thyroid dysfunction in the dog, and can occur in cats with hyperthyroidism.
Typical clinical signs include unprovoked aggression towards other animals and/or people, sudden onset of seizure disorder in adulthood, disorientation, moodiness, erratic temperament, periods of hyperactivity, hypo-attentiveness, depression, fearfulness and phobias, anxiety, submissiveness, passivity, compulsiveness, and irritability. After episodes, most of the animals appeared to come out of a trance-like state, and were unaware of their bizarre behavior.
DIAGNOSTIC TESTING FOR FELINE THYROID DISEASE
Free T4 is the non-protein-bound, physiologically-active form of T4 and therefore the best marker of thyroid status in dogs and cats. Indeed, in cats with over-hyperthyroidism (consistent clinical signs and high T4), the free T4 measured by equilibrium dialysis (ED) is elevated. In cats with early or mild hyperthyroidism. (more mild signs of disease and T4 in the upper half of the normal reference range), the FT4ED is often elevated. However, in 5% to 10% of cats with non-thyroidal illness(e.g., chronic renal failure, IBD, liver disease, neoplasia), FT4ED can be elevated for reasons that are unclear. These cats may have signs consistent with hyperthyroidism,(e.g. weight loss, vomiting) but there is typically no palpable thyroid nodule(s), and T4 values are usually within normal limits.
Testing older cats
- Basal thyroid levels in older cats should be lower than in adults.
- Other illnesses often lower T4, masking hyperthyroidism.
- Minimum testing needed is T4, free T and cTSH.
- FT4ED is usually high in hyperthyroidism but can also be high in inflammatory bowel disease, renal and liver diseases, and neoplasia.
MANAGEMENT AND THERAPY
Dogs
Dogs testing positive for heritable autoimmune thyroiditis should be treated immediately with thyroxine twice daily, even if their basal thyroid levels are still normal (i.e. in the state of “compensative autoimmune thyroiditis”). This is because feedback inhibition of TSHoutput from the pituitary gland shuts off stimulation of thyroid gland receptors, and thus slows or stops thyroid autoantibody production. Thyroxine therapy typically takes around five to seven months or longer, during which thyroid autoantibody levels should gradually decline until they are lower or normal (negative). Regardless, therapy with thyroxine is needed lifelong to prevent the reoccurrence of the genetically controlled autoantibody production.
Keep in mind that thyroxine binds to calcium and soy, so this drug should be given apart from meals (e.g.one hour before in the morning, and three hours after in the evening), regardless of what the product label says. While physicians are taught about the binding behavior of this drug in order to alert their patients, veterinarians traditionally have not been told about it. Because of the heritable nature of this disorder, as discussed above, dogs affected with thyroiditis, even if still asymptomatic, should not be used for breeding purposes.
THYROID DYSFUNCTION IN DOGS — IMPORTANT FACTS
Thyroid dysfunction can affect any dog breed, hybrid/cross-breed, and mixed breed.
Thyroid dysfunction is a heritable and familial trait in many breed types. Accurate diagnosis can be difficult.
- Puppies have higher basal thyroid levels than adults.
- Geriatrics and large/giant breeds have lower basal thyroid levels.
- Sighthounds have much lower basal thyroid levels.
Classical clinical hypothyroidism occurs once the thyroid has undergone 70%+ damage; early signs include weight gain and behavior issues.
T4 alone gives misleading results, despite its common use on health profiles at veterinary reference laboratories; it inaccurately assesses thyroxine therapy, especially in the face of concurrent non-thyroidal illness, and fails to detect autoimmune thyroiditis.
Even T4 plus free T4 and TSH is inadequate for diagnosing heritable thyroiditis.
Canine TSH is poorly predictive of thyroid function (30%), unlike the human test (95%).
In-clinic commercial tests for T4 and even cTSH are inadequate for accurate diagnosis.
Age- and breed-specific norms are essential for accurate diagnosis; Hemopet provides “Case Specific Ranges” on each pet report.
Hemopet thyroid tests are unique, non-RIA and patented.
Thyroxine binds to calcium and soy, so must be given apart from meals.
Twice-daily therapy is preferred, as thyroxine’s half-life is 12 to 16 hours in pets.
Stopping thyroxine to retest the pet’s basal thyroid capacity requires six weeks or more.
Thyroid support/thyrotrophin supplements are inadequate on their own, and natural glandulars are more costly and contain T3.
Cats
After diagnosis, hyperthyroid cats are typically treated with oral (tablet or compounded liquid) or transdermal ear tip applications of methimazole, radioactive iodine-131 therapy to ablate the thyroid acinar cells, or with surgical removal of the thyroid adenoma(s). Six to eight weeks after this therapy, these cats should be retested with at least the T4, T3 and cTSH assays.
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