Hyperthyroidism in Cats: Symptoms, Treatment, and What to Expect
The Most Common Endocrine Disease in Older Cats
Feline hyperthyroidism is the most frequently diagnosed hormonal disorder in cats and one of the most common diseases of middle-aged to older cats overall. It develops when one or both thyroid glands develop a benign tumor (adenoma) that produces excess thyroid hormone. Understanding the signs, the treatment options, and what each option involves helps cat owners make informed decisions about a condition they are very likely to encounter over a cat's lifespan.
How Common Is It?
Hyperthyroidism is rarely seen in cats under 8 years old. In cats over 10, the prevalence is estimated at 10% or more. It has become dramatically more common since the 1970s; the causes likely involve environmental factors including the diet transition to commercial canned cat foods (specific coatings in cans have been implicated) and possibly other environmental contributors. It's now so common that annual thyroid screening is recommended by many veterinarians for all cats over 7-8 years.
Signs and Symptoms
Excess thyroid hormone accelerates virtually every metabolic process in the body, which produces the classic constellation of signs:
Weight loss despite increased appetite: The most classic presentation. Owners notice their cat eating ravenously but losing weight. This combination is highly suggestive of hyperthyroidism.
Increased activity and restlessness: Many hyperthyroid cats become notably more active, sometimes frantic. Yowling, especially at night, is common and can be attributed to the neurological effects of excess thyroid hormone.
Increased thirst and urination: A secondary effect of elevated metabolic rate.
Vomiting and diarrhea: GI signs are common, often from hypermotility of the gut driven by excess thyroid hormone.
Unkempt coat: Cats with hyperthyroidism often decrease grooming, resulting in a rough, matted coat despite increased caloric intake.
Heart changes: Chronic excess thyroid hormone causes hypertrophic cardiomyopathy (thickening of the heart muscle) and hypertension in many cats, which can become serious complications.
Diagnosis
A simple blood test measuring serum T4 (total thyroxine) is the primary diagnostic test. Most hyperthyroid cats have elevated T4. A small percentage ("occult" hyperthyroidism) have T4 in the upper normal range despite clinical disease — these require additional testing (free T4, T3 suppression test, or nuclear scintigraphy).
Treatment Options
Methimazole (Felimazole): The most common initial treatment. A twice-daily medication that blocks thyroid hormone production. Not a cure — if stopped, hyperthyroidism returns. Can cause side effects in some cats (vomiting, facial itching). Available as pills or a topical ear gel (for cats resistant to pilling).
Radioactive Iodine (I-131): The gold standard curative treatment. A single injection of radioactive iodine selectively destroys abnormal thyroid tissue. Over 95% of cats are cured with one treatment. Requires a few days of isolation at a licensed facility (the cat is mildly radioactive for a short period). The most effective long-term option.
Surgical thyroidectomy: Surgical removal of the affected thyroid gland(s). Effective but requires anesthesia in an older cat and carries risk of accidental parathyroid damage causing dangerously low calcium.
Prescription iodine-restricted diet: Hills y/d limits dietary iodine, which is required for thyroid hormone production. Works as a sole treatment if strictly followed, but requires the cat to eat nothing else — challenging in multi-cat households.