Veterinary Information:
Hyperthyroidism in Cats

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Terminology:
Hyperthyroidism in cats: Excessive amounts of circulating thyroid hormones result in multi-systemic disorders. It is the most common endocrine condition in middle-aged to old cats (median age 13 years). Canine hyperthyroidism due to a functional thyroid tumor is fairly uncommon.

Aetiology:
Hyperthyroidism in dogs and cats is generally caused by functional neoplasias or hyperplasias.
In cats these are generally multiple thyroid adenomatous hyperplasias, while thyroid carcinomas are the main cause of this condition in hyperthyroid dogs. In approximately 70% of the hyperthyroid cats, both thyroid lobes are involved. Thyroid carcinomas are very rare in cats with a prevalence of less than 2%. Hypothetical causes of these adenomatous hyperplasias are genetic factors, circulating immunoglobins, nutritional or environmental factors (iodine or toxins).

In humans a genetic cause for multinodular hyperthyroidism (equivalent to feline adenomatous thyroid hyperplasias) is identified. A mutation in the TSH-receptor gene and its associated G-protein result in a continues stimulation of the cyclic AMP signalling pathway, resulting in hyperthyroidism and hyperplastic adenomatous changes in the thyroid glands.

A similar mutation of genes involved in the thyroid hormone synthesis is likely to play a crucial role in the pathogenesis of hyperthyroidism in cats. These gene mutations could either cause an increased stimulation or a decreased inhibition of the thyroid hormone synthesis.

Predisposition:
Age: mostly middle-aged or older cats, ranging from 4-23 years (median age 13 years, 95% older than 10 years).
Both in males and females.
Breeds: There is no clear breed predilection, although Siamese and Himalayan cats seem to have a diminished risk on becoming hyperthyroid.

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