Feline Hyperthyroidism

Introduction
Feline hyperthyroidism is a relatively common disease of middle-aged and elderly cats, with a mean age of 12 to 13 years. there is no breed or sex predilection. the thyroid hor- mone excess is produced by thyroid adenomatous hyperplasia or adenoma, involving one or, more often, both thyroid lobes. thyroid carcinoma, which is the main cause of hyperthyroidism in dogs, accounts for only about 5% of cases.

Clinical manifestations
The adenomatous glands tend not to become very large, so rarely is veterinary help sought because of a mass detected by the owner. thus it is the signs and symptoms due to effects of thyroid hormone excess on organ systems that lead to veterinary examina- tion. the classic presentation of a hyperthyroid cat is that of a skinny, restless, elderly cat with an increased appetite and polyuria. Many organ systems can be affected. Weight loss—often together with increased appetite—may be sufficient reason to suspect hyperthyroidism. in about 10% of cases the clinical picture may be quite different. in these cats weight loss remains an important feature, but there is lethargy and anorexia rather than hyperactivity and increased appetite.

The multisystemic effects of thyroid hormone excess not only lead to a variety of physi- cal changes but may also give rise to several biochemical abnormalities. Most of these are reversed with treatment, including elevated plasma concentrations of liver enzymes. the hemodynamic alterations of hyperthyroidism are responsible for marked increases in the glomerular filtration rate. of concern has been the increase in the plasma creati- nine concentration after treatment of hyperthyroidism, although it is often still within the reference range. although considered to be the unmasking of preexisting chronic kidney disease, it seems to have little clinical significance. the survival of treated hyper- thyroid cats does not seem to be affected by post-treatment azotemia. [...]