Pharmacotherapeutic group:antithyroid preparations, sulphur-containing imidazole derivates.
ATC Vet 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 particulars
Absorption
Following oral dosing in healthy cats, thiamazole is rapidly and completely absorbed with a bioavailability of >75 %. However, there is a considerable variation between animals.
Peak plasma levels occur approximately 0.5-1 hour after dosing (tmax = 0.69 h). Cmax is between 1.1 and 2.7 µg/ml (1.78 µg/ml) and half-life is 3.3 h.
Distribution
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.
The drug residence time in the thyroid gland is assumed to be longer than in the plasma.
Metabolism and elimination
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.