Thiamazole, 1-methyl-imidazole-2-thiol, is a white, crystalline substance that is freely soluble in water. It differs chemically from the drugs of the thiouracil series primarily because it has a 5- instead of a 6-membered ring. The molecular weight is 114.16 and the empirical formula is C4H6N2S. It is an orally administered antithyroid drug.