Hyperthyroidism is a disease caused by an overactive thyroid gland, an organ found on either side of the windpipe at the base of the neck. This gland produces thyroid hormone which helps to regulate your cat's metabolism, or rate of bodily activity. When the thyroid gland produces too much hormone, your cat's 'internal motor' effectively goes into overdrive. Untreated this would eventually be fatal but the condition can now be successfully treated.
Hyperthyroidism was first seen in cats as recently as 30 years ago and appears to be caused by a form of benign cancer in the thyroid gland. However, it is still not clear what causes the cancer to develop. The disease is rare in young cats but becomes more common in later life. It is now the most frequent hormonal disease in middle-aged and older cats.
The first indication that anything is wrong is usually a marked increase in your cat's appetite. Even though your cat is eating more it may lose weight and its coat may become rough and unkempt. Other changes include restlessness and aggression, body tremors, increased drinking and peeing, vomiting and diarrhoea. In about one case in ten the symptoms are unusual and the opposite of what might be expected, such as depression, loss of appetite and physical weakness.
When your vet examines your cat's throat the thyroid gland may feel lumpy or enlarged. Blood tests are usually taken to rule out other diseases of the liver or kidneys. Directly measuring levels of hormone in the blood may help confirm the diagnosis but in some cats the thyroxine levels may be normal. Your vet will also want to check your cat's heart - an abnormally fast or irregular heart beat is often a feature. Early diagnosis and treatment is important to prevent and even reverse damage to the heart and kidneys.
There are drugs available which block the production of hormones by the thyroid gland. The medication is given one to three times a day.The abnormal gland can be surgically removed. Treatment should permanently cure the disease so no need for further medication. Not suitable for all cats, such as those with severe kidney disease or the very elderly.
( from Vetstream Felis)