Hyperthyroidism is common in older cats and is often caused by a benign tumor called an adenoma (99.9%). Far less frequently (0.01%), it is caused by a malignant tumor. The tumor causes excessive production and secretion of thyroid hormone which can result in life threatening conditions.
The most common clinical signs include;
hyperactivity
behavioral changes
weight loss
vocalization
increased appetite
poor hair coat
increased heart rate
vomiting
heart arrythmia diarrhea increased drinking
increased urination
Less common clinical signs include;
decreased appetite
decreased activity
weakness
labored breathing
large fecal volume
If the hyperthyroidism is not treated, the cardiac disease that develops can be fatal. Fortunately, hyperthyroidism is treatable and can be cured.