Hyperthyroidism is the most common endocrine disease in cats over 8 years. Use the Bloodwork OCR to interpret thyroid panels and the Endocrinology Specialist for management guidance.
Hyperthyroidism results from excessive production of thyroid hormone (T4, T3) by the thyroid gland, usually due to benign adenomatous hyperplasia or adenoma. It is the most common endocrine disease in cats, typically affecting those over 8 years. The excess hormone accelerates metabolism, affecting the heart, kidneys, and overall condition.
Classic signs include weight loss despite a good appetite, hyperactivity, increased thirst and urination, vomiting, diarrhea, unkempt coat, and tachycardia. Some cats present with heart murmurs or hypertension. The disease can mask early chronic kidney disease—kidney values may appear normal until the hyperthyroidism is controlled.
Total T4 is the initial screening test. Most hyperthyroid cats have elevated T4. In borderline cases or when T4 is normal but clinical suspicion is high (e.g., concurrent illness suppressing T4), free T4 or T3 suppression test may be used. Free T4 by equilibrium dialysis is more sensitive but can have false positives in sick euthyroid cats. Thyroid scintigraphy can confirm and localize functional thyroid tissue.
| Treatment | Pros | Cons |
|---|---|---|
| Methimazole (oral or transdermal) | Reversible; no anesthesia; inexpensive | Lifelong; side effects (vomiting, lethargy, facial itching); twice-daily dosing |
| I-131 (radioactive iodine) | Curative; single treatment; no daily meds | Requires isolation; expensive; specialized facility |
| Thyroidectomy (surgery) | Curative; no radiation | Anesthesia risk; hypocalcemia if parathyroids damaged; recurrence if ectopic tissue |
| Prescription diet (iodine-restricted) | No medication; palatable for some | Must eat exclusively; not for cats with access to other food; not curative |
Methimazole blocks thyroid hormone synthesis. Starting dose is typically 2.5-5 mg twice daily. Recheck T4 in 2-4 weeks and adjust. Transdermal formulations (compounded gel applied to ear pinna) are an option for cats that resist pills. Side effects include vomiting, anorexia, lethargy, and rarely blood dyscrasias—monitor for signs of illness.
Radioactive iodine is taken up by thyroid tissue and destroys it. It is curative in most cases. Cats must be hospitalized for several days until radiation levels drop below regulatory limits. Hypothyroidism can occur afterward and may require supplementation.
Regardless of treatment, monitor T4 and kidney values regularly. After methimazole or I-131, some cats develop or unmask kidney disease—dose adjustment or treatment may be needed.
Early detection allows for appropriate treatment and monitoring of concurrent conditions like kidney disease.
Weight loss, increased appetite, hyperactivity, vomiting, or increased thirst in an older cat should prompt a veterinary visit and thyroid screening.
- Hyperthyroidism is most common in cats over 8; weight loss despite good appetite is classic.
- Total T4 screens; free T4 or T3 suppression used in borderline cases.
- Methimazole is reversible; I-131 is curative; surgery and diet are alternatives.
- Monitor T4 and kidney values; hyperthyroidism can mask kidney disease.
- Early screening enables appropriate treatment and concurrent condition management.