Methimazole transdermal gel is a compounded antithyroid preparation applied to the inside of a cat's ear flap instead of given by mouth. The drug absorbs through the thin skin of the pinna. A veterinary prescription is required. Felimazole remains the FDA approved oral tablet for feline hyperthyroidism.
How It Works
The pinna is thin, lightly haired, and easy for an owner to reach, which is why it became the standard site for feline transdermal dosing. A small measured amount of gel is rubbed into the inner surface, alternating ears to limit local irritation.
The base does most of the work. Early preparations used pluronic lecithin organogel. Pharmacokinetic work in healthy cats found methimazole absorption from PLO was low and inconsistent, with detectable serum levels in only two of six cats in the first 24 hours (Hoffman SB, Yoder AR, Trepanier LA. J Vet Pharmacol Ther. 2002;25(3):189-193).
Clinical outcomes told a more favorable story. In a randomized trial of 47 newly diagnosed hyperthyroid cats, transdermal methimazole in PLO applied to the inner pinna was compared against oral methimazole at 2.5 mg every 12 hours (Sartor LL, Trepanier LA, Kroll MM, Rodan I, Challoner L. J Vet Intern Med. 2004;18(5):651-655. doi:10.1111/j.1939-1676.2004.tb02601.x). Longer follow up has since been published on transdermal use in hyperthyroid cats (Boretti FS, Sieber-Ruckstuhl NS, Schäfer S, et al. J Feline Med Surg. 2014;16(6):453-459. doi:10.1177/1098612X13509808).
Absorption varies between individual cats. That variability is exactly why recheck bloodwork and veterinary follow up matter more with a transdermal than with a tablet.
Who Prescribes It
Small animal general practitioners, feline only practices, veterinary internal medicine, and mobile or house call veterinarians. Hospital practice managers typically place the order once the veterinarian selects the strength and base.
The common trigger is administration failure. A cat that spits pills, an owner with limited hand dexterity, a fractious patient, a multi cat household where dosing gets confused, or a cat the owner has simply stopped medicating. Twice daily oral dosing over years is a compliance problem, not a pharmacology problem, and the dosage form is where it gets solved.
Formulation Options
- Strength written per metered dose, typically 0.05 mL or 0.1 mL
- Base selected by the prescriber, including PLO or a lipophilic permeation enhancing base
- Metered dose pen or graduated syringe dispensing
- Dye free and flavor free preparation
- A second active reviewed case by case for chemical compatibility before compounding
Quality and Compliance
Methimazole appears on the NIOSH List of Hazardous Drugs in Healthcare Settings, the list referenced by USP General Chapter 800. Mixwell prepares it inside a purpose built negative pressure hazardous drug room under USP 795 and USP 800, which is the relevant question when a practice is deciding where to send the prescription. Mixwell is non sterile only and prepares every veterinary formulation pursuant to a patient specific prescription. Finished preparations are delivered across California.
Compounded preparations are not FDA approved for safety or effectiveness. Speak with your veterinarian about your individual patient before starting or changing any medication. Veterinary prescriptions cannot be charged to HSA or FSA accounts.
More on Mixwell's veterinary compounding services, the PCCA bases used in Mixwell formulations, how to write a compounded prescription in California, and the pet owner guide to transdermal methimazole.
Reviewed by Linda Pungpravat, Compounding Pharmacist, RPH, Mixwell Compounding Pharmacy, Chino, CA.