As pharmacy continues to evolve, so does our knowledge of how to treat animals. My experience working at the University of Tennessee Vet Hospital exposed me to the number of problems animals face and the struggles their owners go through to take care of them.

Methimazole is commonly used in hyperthyroidism in cats. You can imagine the issues you have delivering medication to a cat orally especially one that is sick. Another avenue of medication delivery is through a gel. Pluronic is a popular base, however we use lipoderm as we have found better penetration through the tissue . Lipoderm allows penetration through the skin of the patient, preferably we try to use the inside of the ear.  Also, milling the cream as well as using an unguator should be used to ensure particle size is as small as possible. The dosage required to achieve control varies and is applied once or twice daily in most cats.

As a transdermal agent, make sure to wear gloves and discard appropriately when applying as it can absorb through your skin as well. Make sure to followup with your Veterinarian regularly to ensure the dosage prescribed is obtaining the clinical effect

A group of researchers from the Vetsuisse Faculty at the University of Zurich looked at short and long-term follow-up in a group of cats treated with transdermal methimazole. The study encompassed 20 newly diagnosed hyperthyroid cats. Initial dose was 2.5 mg BID. All cats were rechecked between one and four weeks after starting therapy. Eight cats were also reevaluated at five to eight weeks, nine to 20 weeks and after a median of 42.5 weeks (range 21 to 118 weeks). This study also attempted to look at the effect of the gel on T4 concentrations over a 10-hour period (sampling every two hours after administration). This was carried out in six cats on the first day of treatment and one week after starting therapy. In three cats, this was also done three weeks after starting therapy.

Treatment was effective in controlling clinical signs. Side effects were rare and included GI signs in two cats and ear irritation in one cat. Significant decreases in T4 were noted at the first recheck and at every time point afterward. Some cats were still in the hyperthyroid range except at the last recheck where the highest T4 concentration was 46.3 nmol/L (reference range generally goes to 45).

In order to achieve this, methimazole dosage had to be increased to 3.5 to 8.7 mg per application in nine cats. There were no significant changes noted in T4 concentration during the 10 hours it was sampled.

The authors concluded that methimazole in PLO gel is effective at treating hyperthyroidism long-term. The lack of detectable effect on T4 over the 10-hour period after administering the gel suggests that once-a-day dosing may be adequate, although this information has yet to be confirmed.

Published data to date shows that transdermal methimazole can be an effective medication to control hyperthyroidism. It appears to take longer to achieve control than with oral medications and a higher dosage of methimazole may be needed. Long-term management is possible and appears to be well-tolerated by cats. Adverse side effects can occur and are similar to those seen with oral administration, though some studies suggest that GI signs are less likely to be present.

Resting Cat

*image credit:


1. Sartor LL, Trepanier LA, Kroll M, et al. Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidism. J Vet Intern Med 18: 651-655: 2004.
2. Boretti FS, Sieber-Ruckstuhl NS, Tschour F, et al. Short and long-term follow-up of hyperthyroid cats treated with transdermal methimazole. J Vet Intern Med 20;1523-1524: 2006.