Hyperthyroidism is the most common endocrinopathy of middle-aged and older cats and is most often the result of excessive circulating levels of thyroxine (T4) and triiodothyronine (T3) produced by a thyroid adenoma or hyperplastic adenomatous thyroid tissue.24-28 Thyroid carcinoma as a cause of hyperthyroidism is rare.24
Clinical signs include weight loss (despite polyphagia), hyperactivity, polyuria/polydipsia, unkempt hair coat, GI signs (eg, vomiting, diarrhea), panting/dyspnea, lethargy or mental dullness, neuromuscular weakness, anxiety, nervousness, and/or behavior changes. Physical examination may reveal a thin cachectic cat, a palpable thyroid nodule, a heart murmur, tachycardia, arrhythmia, tachypnea, alopecia or unkempt hair coat, aggression, or mental dullness.24-28 Cervical flexion and generalized muscle weakness occur in some cats with hyperthyroidism. Hyperthyroidism can also cause a vascular lesion in the cervical spinal cord of some cats that results in cervical flexion. In a study, concurrent hypokalemia was reported in 4 hyperthyroid cats with cervical flexion,29 but the cause of hypokalemia was undetermined.
Diagnosis for most cats is made through measurement of elevated total T4 (TT4) by radioimmunoassay, but ≤10% of hyperthyroid cats have normal TT4 during initial testing; these cats are in the early stage of disease and either TT4 has not risen above the high-end normal limit or nonthyroidal illness is artificially lowering the TT4 level.24,25,30-33 Free T4 should not be used as the sole screening test for hyperthyroidism because it is more sensitive and less specific than TT4, causing more false-positive results.31,34
Treatment options include medical management with thioureylene antithyroid drugs (eg, methimazole), low iodine diet, radioactive iodine (I-131) therapy, and thyroidectomy.35 Methimazole (2.5 mg/cat PO every 12 hours) is most frequently used in the United States.35 Serum TT4 is measured 4 weeks later and the dose is gradually increased or reduced based on the results of repeated TT4 testing.35 Adverse effects include GI effects (eg, vomiting, anorexia), nonspecific signs (eg, depression, lethargy), blood dyscrasias (eg, thrombocytopenia, leukopenia, eosinophilia, lymphocytosis), pruritus and facial excoriations, and hepatopathy (rare).35 It is recommended that medical management with methimazole be attempted prior to more advanced treatment in order to screen for underlying CKD, which can be masked by hyperthyroidism secondary to increased glomerular filtration rate and decreased serum creatinine concentration.35
I-131 is the most effective option and the treatment of choice; it is typically safe and simple to administer but only available at specialized licensed facilities.35 I-131 is concentrated in the thyroid glands, where it emits β particles and γ radiation that destroy functional hyperplastic thyroid tissue.35 Thyroidectomy can be curative and may be performed when I-131 is not readily available.35 Anesthesia can cause the secondary metabolic, renal, and cardiac changes to elevate the American Society of Anesthesiologists status and increase the risk for perioperative complications. Thyroidectomy is required for patients with thyroid carcinoma (rare).35