Hyperthyroidism is a common hormonal imbalance in older cats. Dogs are rarely affected by this condition.
So, what causes it? Hyperthyroidism is caused by a benign growth in the thyroid glands - located at each side of the windpipe , which results in overproduction of the hormone thyroxine (T4). T4 is converted in the body cells to T3 (triiodothyronine), which is the active form of the hormone. It is important to realise that these growths are almost always benign and represent a form of goitre rather than a form of cancer.
Only 3% to 5% of hyperthyroid cats have a cancerous thyroid growth. Thyroxine is an important hormone that controls the rate of metabolism(the biochemical processes necessary for life) and so impacts on body systems. An increase in levels results in an increase in metabolic rate, heart rate, and blood pressure. Despite an increase in appetite (polyphagia), weight loss occurs and diarrhoea may be seen.
Affected cats may be restless and hyperactive in behaviour. In some cases vomiting, increased thirst (polydipsia) and urination (polyuria) may be seen. In advanced cases, the signs may be the complete opposite, with lethargy and weakness shown.
See more in the following links http://www.aht.org.uk/pdf/feline_hyperthyroidism.pdfhttp://www.merialvetsite.com/sites/barnfieldvets/313hyperthyroidism.pdfhttp://www.gla.ac.uk/faculties/vet/smallanimalhospital/ourservices/internalmedicine/radioiodineunit/