Use of a Controlled-Release Formulation of Carbimazole for the Treatment of Feline Hyperthyroidism

Rosenburg D., Frenais R., Fornel-Thibaud P., et al.

Conference Proceedings, (2008). Proceedings of the British Small Animal Veterinary Association Annual Congress: p.488


Carbimazole and its active metabolite methimazole are the standard medical treatment for hyperthyroidism in cats. Most effective protocols are based on administration two or three times a day. A novel controlled-release formulation of carbimazole suitable for once daily administration was evaluated in cats newly diagnosed with hyperthyroidism in a multicentre, self-controlled study.

Client-owned cats (n=44) with history and clinical signs consistent with hyperthyroidism, confirmed by a total thyroxine concentration (tT4) >50 nmol/L, were administered controlled-release carbimazole tablets (15 mg/10 mg) once daily. Treatment was started at a dose of 15 mg and reassessed at four subsequent visits after 10 days, and 3, 5 and 8 weeks. Thereafter, cats could remain on treatment for up to 12 months, with an additional visit at 26 weeks. The response to treatment was evaluated based on clinical signs and tT4.

Median tT4 was 118 nmol/L (range 50-320 nmol/L) at inclusion. Median tT4 was 33 nmol/L at 10 days, 27 nmol/L at 8 weeks and 28 nmol/L at the end of the study. A subjective classification of euthyroidism was made by the investigators, based on their clinical evaluation (body weight gain, reduction in heart rate, and improved appetite, thirst and attitude/behaviour), in 16/40 cats within 3 weeks, 31/40 cats at 8 weeks and 22/24 cats by the end of the study. This was confirmed by a median tT4 <50 nmol/l in 28/40 cats at 3 weeks, 30/40 cats at 8 weeks and 16/24 cats at the end of the study. In the cats where tT4 remained >50 nmol/L, tT4 decreased by 56% (median, range 26-84%) at 8 weeks and 69% (median, range 15-73%, except one cat +57%) at the end of the study. The median dose of carbimazole was 10 mg (range 5-20 mg) once daily at 8 weeks and 15 mg (range 5-25 mg) once daily at the end of the study. A probable treatment-related adverse reaction was reported in one case (facial pruritus that improved within 10 days following appropriate symptomatic treatment), which remained on carbimazole treatment.

Once daily administration of the controlled-release carbimazole tablets was effective in and well tolerated by most of the hyperthyroid cats during short- and long-term treatment.