Hyperthyroidism is the most common endocrine disorder in cats. This disorder has been noted with increased frequency since the late 1970's. It also appears to be more prevalent in certain geographic locations.

Normal Thyroid Physiology

The hypothalamus releases thyrotropin-releasing hormone (TRH), which causes the pituitary gland to release thyrotropin-stimulating hormone (TSH). TSH increases production and release of thyroxine (T4) and triiodothyronine (T3), and reverse T3 (rT3) from the thyroid gland. Increased serum T4 is the major factor that inhibits further release of TSH from the pituitary gland. The majority of hormone produced is T4; however it is T3 that is active at the tissue level. T4 is converted to T3 within the tissues. In circulation T3 and T4 are highly bound to proteins, and it is only the free hormone that is available for biological activity. Thyroid hormones affect virtually every tissue in the body. Thyroid hormones increase overall metabolic rate and stimulate protein synthesis and breakdown, carbohydrate metabolism, and fat utilization. Other effects include stimulation of erythropoiesis, formation and reabsorption of bone, increased mitochondrial oxygen consumption, increased heart rate and strength of contraction.

The clinical signs associated with hyperthyroidism are a direct result of altered metabolism by excessive amounts of thyroid hormone (T4 and T3) on the target tissues (thyrotoxicosis). Hyperthyroidism in cats is generally a primary disease, meaning that the thyroid tissue is functioning autonomously, and is not secondary to a pituitary or hypothalamus disease.

This disease is generally caused by benign thyroid neoplasia or adenomatous hyperplasia of one (15-20%) or both (70%) thyroid lobes. Thyroid carcinoma is very uncommon (less than 3%). Ectopic thyroid tissue occurs in approximately 3%. There are many theories as to the underlying cause, but none has been proven to be the definitive etiology.

Signalment

Cats range from 4-20 years old, but 95% are >8 years old. Purebred cats are less likely to become hyperthyroid than domestic /mixed breeds. There is no gender difference.

History

The main owner complaints generally include some of the following: polydipsia, polyuria, polyphagia, weight loss, hyperactivity or nervousness, vomiting/diarrhea, tachypnea/panting, weakness, lethargy, decreased grooming, heat avoidance.

Physical Exam Findings

Examination generally reveals one or all of the following: poor body condition, unkempt haircoat, dehydration, tachycardia, arrhythmias/murmurs, abnormal retinal examination (hypertension), and palpable thyroid glands.