Hyperthyroidism — overproduction of thyroid hormone by the thyroid glands — is the most common endocrine disorder in cats, affecting an estimated 10% of cats over ten years of age. It is a serious condition that, if left untreated, causes progressive weight loss, heart problems, high blood pressure, and eventually death. The good news: it is highly treatable, and with proper management, most cats make a full recovery.
What Causes It
The vast majority of feline hyperthyroidism cases are caused by a benign tumor of the thyroid gland called an adenoma that produces thyroid hormone uncontrollably. A small percentage of cases (about 2%) involve a malignant tumor called thyroid adenocarcinoma. The exact cause is unknown, though potential risk factors include dietary iodine content, exposure to certain flame-retardant chemicals (polybrominated diphenyl ethers or PBDEs), and canned food consumption — though the evidence for dietary factors is not conclusive.
Symptoms
The classic presentation is an older cat who is losing weight despite a ravenous appetite. Other signs include increased thirst and urination, hyperactivity and restlessness (or conversely, lethargy), vomiting, diarrhea, poor coat quality, and a matted, greasy-looking coat. Some cats develop a rapid heart rate or heart murmur. Diagnosis is confirmed with blood work showing elevated thyroid hormone (T4) levels.
Treatment Options
Treatment options include medication (methimazole, which blocks thyroid hormone production), radioactive iodine therapy (the gold standard cure — a single injection that destroys the tumor), surgical thyroidectomy, and dietary therapy (prescription iodine-restricted food). Radioactive iodine therapy has a success rate above 95% with minimal side effects and is considered the best long-term cure for most cats.