Thyroid glands: diseases and laboratory diagnosis in small animals

Thyroid diseases are among the most common endocrine disorders in small animals. Hypothyroidism is a common disease in dogs, but spontaneous hypothyroidism is very rare in adult cats. Hyperthyroidism is one of the most common diseases of cats and is uncommon in dogs. Hypothyroidism is primarily a disease of middle aged to old dogs with clinical signs including weight gain to obesity, lethargy, dull haircoat, cold intolerance detected as heat-seeking behavior, decreased libido, reproductive failure, alopecia with no pruritus, and hyperpigmentation in areas of alopecia. Laboratory abnormalities may include mild anemia, increased liver enzymes and increases in muscle enzymes (CPK). Hypertriglyceridemia and hyperlipidemia occurs in a majority of cases. Hypercholesterolemia is seen in approximately 80% of hypothyroid dogs and a serum cholesterol concentration greater than 500 mg/dL is very suggestive of hypothyroidism. Basal concentration of total T4 should be the initial endocrine diagnostic test utilized when hypothyroidism is suspected. However, approximately 20% of dogs without hypothyroidism may also have decreased TT4. In addition, total T4 may be in the normal range in about 10% of dogs with hypothyroidism. Therefore, it is important to measure other endocrine tests (free T4 and TSH concentrations). The challenging cases may require more intensive diagnostic procedures such as repeat testing in 4 weeks and/or stimulation tests (TSH or TRH). Hyperthyroidism is the most common endocrine disease of cats. Hyperactivity, weight loss, and polyphagia in a middle aged to old cat are the most frequent clinical problems. Increase in one or more liver enzymes, azotemia, hyperphosphatemia and erythrocytosis are the most consistent lab abnormalities of the hyperthyroid cats. If a cat has some of the physical and clinical laboratory abnormalities characteristic of hyperthyroidism, and an increased TT4 concentration, it is diagnostic of hyperthyroidism and fT4 or any additional tests are not needed. When faced with conflicting clinical signs and lab data while trying to confirm a diagnosis of hyperthyroidism, other endocrine tests such as repeating total T4 in 1-2 weeks, free T4 concentration, T3 suppression test and/or stimulation tests (TSH or TRH) should be considered.