Introduction
Hyperthyroidism in cats occurs when the thyroid gland begins to produce too much thyroid hormones. The increase in the production of these thyroid hormones is due either to an increase in the size of the thyroid or a type of thyroid cancer.
Hyperthyroidism generally affects middle to older aged cats.
Causes of Hyperthyroidism in Cats
Enlarged Thyroid Gland
Most of the cases of hyperthyroidism in cats, well over 95% of them, are caused by an enlargement of either one, or both, lobes on the thyroid gland. This enlargement is caused by benign growths which occur on the thyroid gland when cell division on the thyroid gland malfunctions and begins to rapidly increase.
As the thyroid increases in size, it begins to produce increased amounts of thyroid hormones. The two thyroid hormones that are increasingly produced, T4 (thyroxine) and T3 (triiodothyronine), are responsible for vital body functions including metabolic rate, protein synthesis, and the health of cells. High levels of these thyroid hormones disrupt these functions, and cause the symptoms that are associated with hyperthyroidism.
What causes these benign growths to occur is not known, however some of the suspected causes of the growths are genetics, diet, and conditions in the immune system. The growths may occur just on one lobe, but in over 70% of hyperthyroidism cases in cats both lobes are affected.
Thyroid Cancer
In a very small amount of hyperthyroid cats, the condition is caused by a thyroid carcinoma also known as a thyroid cancer. This condition occurs when a malignant growth occurs on one, or both, of the thyroid glands.