Hyperthyroidism in Cats
Feline hyperthyroidism is the most common endocrine disorder in cats over 10 years old, caused by benign overproduction of thyroid hormone (thyroid adenoma). Untreated, it leads to weight loss, secondary cardiomyopathy, and hypertensive end-organ damage. The 4 evidence-based treatments are methimazole (oral or transdermal), radioactive iodine (I-131), thyroidectomy, and prescription iodine-restricted diet.
Carbimazole and methimazole can cause facial pruritis (1-3% of cats) and hepatopathy. Baseline + 2/4/8/12-week CBC/chem/T4 monitoring is the AAFP/ISFM standard.
Symptoms to watch for
- •Dramatic weight loss despite increased appetite
- •Polydipsia and polyuria (drinking + urinating more)
- •Restlessness, vocalization, especially at night
- •Poor coat condition, increased shedding
- •Tachycardia or new heart murmur on exam
Treatment options
Standard of care for daily medical management. Compounded transdermal gel applied to inner ear pinna avoids twice-daily pilling and is preferred by ISFM for cats that refuse tablets.
Pro-drug of methimazole; used widely outside the US.
Why compounded medication matters
Methimazole transdermal gel (compounded) is preferred over commercial tablets for chronic feline hyperthyroidism management when oral dosing is unreliable. The pinna application bypasses pill aversion and is well-tolerated long-term per the 2016 ACVIM Consensus Statement.
See compounded methimazole optionsValid vet prescription required · HIPAA-compliant · Free to pet owners
FAQs — Hyperthyroidism in Cats
Sources & review
Reviewed by: ScriptUnlock Pharmacy Verification Team
Last reviewed: June 2026
Sources cited:
- • ACVIM Consensus Statement on Hyperthyroidism (2016)
- • ISFM Feline Hyperthyroidism Guidelines
- • Plumb's Veterinary Drug Handbook 10th ed.
This information is educational and does not substitute for veterinary advice. Always confirm diagnosis, dosing, and treatment plan with your veterinarian. ScriptUnlock connects pet owners to verified pharmacies; we do not provide veterinary advice.