Hyperthyroidism in cats

Hyperthyroidism in CatsVeterinary & Aquatic Services Department, Drs. Foster & Smith, Inc

Hyperthyroidism is the most common endocrine (hormone) disorder that affects cats. It creates a wide range of signs resulting from the overproduction of thyroid hormone made by the thyroid gland.

What is the thyroid gland?
The thyroid gland is a small and consists of two lobes, one on each side of the trachea (windpipe) in the neck. This gland produces the major thyroid hormone called thyroxine (T4) and a small amount of another hormone, triiodothyronine (T3). These hormones regulate the body's metabolic rate and affect every system in the body. The production of the thyroid hormones is controlled by the hormone called thyroid stimulating hormone (TSH). TSH is produced by the pituitary gland, which is found at the base of the brain.

What causes hyperthyroidism?
If the thyroid gland produces excess amounts of the thyroid hormones, the condition called hyperthyroidism results. The most common cause is benign (non-cancerous) increases in the number of cells in the thyroid gland. Groups of these abnormal cells form small nodules on the thyroid gland and are termed adenomas. Multiple adenomas may form in the same lobe, and in approximately 70% of the cases, both lobes are involved. Only 1-2% of hyperthyroid conditions in cats are caused by malignancy (cancer).

The incidence of hyperthyroidism in cats has increased remarkedly in the last 25 years. The reason for this is unknown, but probably due to multiple factors. The ingredients and types of foods fed, immunological factors, and environmental influences may be involved.

Which cats are most likely to become hyperthyroid?
Hyperthyroidism occurs most commonly in middle to old-age cats with a reported range of onset between 4 and 22 years. The median age for acquiring the disorder is just under 13 years. Only 5% of hyperthyroid cats develop the disease before 8 years of age. There does not appear to be a breed or sex predilection.

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