Pharmacotherapeutic group: Antithyroid preparations: Sulphur-containing imidazole derivatives.
ATCvet code: QH03BB02
Pharmacodynamic properties
Thiamazole acts by blocking the biosynthesis of thyroid hormone in vivo. The primary action is to inhibit binding of iodide to the enzyme thyroid peroxidase, thereby preventing the catalysed iodination of thyroglobulin and T3 and T4 synthesis.
Pharmacokinetic properties
Felimazole 1.25 mg and 5 mg coated tablets for cats:
Following oral dosing in heathy cats, 5 mg thiamazole is rapidly and completely absorbed with a bioavailability of >75%. However, there is a considerable variation between animals. Elimination of the drug from cat plasma is rapid with a half-life of 4.5-5.0 hours. Peak plasma levels occur approximately 1-2 hours after dosing. Cmax is between 1.6-1.9 μg/ml.
Felimazole 2.5 mg coated tablets for cats:
Following oral dosing in heathy cats, thiamazole is rapidly and completely absorbed with a bioavailability of >75%. However, there is a considerable variation between animals. Elimination of the drug from cat plasma is rapid with a half-life of 3.5-4.0 hours. Peak plasma levels occur approximately 1-2 hours after dosing. Cmax is approximately 0.8 μg/ml.
All strengths:
In rats thiamazole has been shown to be poorly bound to plasma protein (5 %); 40% was bound to red blood cells. The metabolism of thiamazole in cats has not been investigated, however, in rats thiamazole is rapidly metabolised in the thyroid gland. About 64% of the administered dose being eliminated in the urine and only 7.8% excreted in faeces. This is in contrast with man where the liver is important for the metabolic degradation of the compound. The drug residence time in the thyroid gland is assumed to be longer than in the plasma.
From man and rats it is known that the drug can cross the placenta and concentrates in the foetal thyroid gland. There is also a high rate of transfer into breast milk.