What causes hyperthyroidism?

Hyperthyroidism is the most common endocrine disorder occurring in middle-aged and older cats. It is caused by the over-production of thyroid hormones from the thyroid gland.

The thyroid gland is located in the neck. In the cat, the gland is divided into 2 lobes - one lobe lies on either side of the windpipe (trachea).

The thyroid gland produces 2 hormones: thyroxine and triiodothyronine, also known as T4 and T3.

Hyperthyroidism occurs when either one or both thyroid lobes enlarge. As a result, the production of thyroid hormones increases and eventually becomes excessive. Exactly why the thyroid lobe(s) start to enlarge is not known. Fortunately, in over 98% of cases, it is a benign (non-cancerous) change and treatment is very successful. Less than 2% of cases are caused by a malignant (cancerous) growth of the thyroid gland.