Hyperthyroidism is one of the most common endocrine diseases affecting older cats. Hyperthyroidism typically results from a functional thyroid adenoma (98%) or a carcinoma (1-2%). In the euthyroid cat the thyroid hormones are released from the thyroid gland in response to thyroid stimulating hormone (TSH). The hormones circulate as thyroxine (T4) and the biologically more active triiodothyronine (T3). Thyroxine (T4) acts as acirculatingreservoir of thyroid hormone and is converted to T3 in response to concentrations of T3.
Functional neoplastic tissue breaks away from the negative feedback loop and overproduces thyroid hormones despite low concentrations of TSH. The thyroid glands are bi-lobar and located on either side of the trachea, normally distal to the larynx. There are two associated parathyroid glands. One is extracapsular and situated adjacent to the thyroid capsule at the cranial pole while the other is surrounded by the thyroid tissue and is found at the caudal pole. Cranial pole of the capsule a branch of the carotid, the cranial thyroid artery, supplies each thyroid gland. Recurrent laryngeal nerves run close to the thyroid glands.
The epidemiology of hyperthyroidism in cats remains unclear and no single underlying cause has been identified. It is likely that diet and the environment play a role. In humans, hyperthyroidism is twice more likely to occur in humans with dietary iodine deficiency. However the pathophysiology of hyperthyroidism development in humans is different humans develop autoantibodies against the TSH receptor, whereas this is not seen in cats. The second most common cause of human hyperthyroidism is toxic nodular goitre one or more hyperfunctioning adenomas similar to cats...