In the US, two antithyroid medications are being used, propylthiouracil (PTU) and methimazole, while in Europe and Asia, carbimazole which has a similar action to methimazole, is being used (Fenton 2006a). Carbimazole is converted to methimazole inside the body (Ross 2008a). The three antithyroid medications mentioned belong to the class of compounds known as thionamides and these medications inhibit thyroid hormone biosynthesis by decreasing the oxidation of iodide and iodination of tyrosine (Fenton 2006b). PTU also blocks the conversion of T4 (thyroxine) hormone to the more metabolically active T3 (triiodothyronine) hormone (MedicineNet 2005a).
The chemical IUPAC name of methimazole is 1-methyl-3H-imidazole-2-thione, its chemical formula is C4H6N2S, its average molecular weight is 114.1688 g/mol, its melting point is 146 degrees Celsius, its experimental water solubility is 275g/L, and it has a half life of five to six hours (DrugBank 2007a). The following statements describe the mechanism of action of methimazole:
'Methimazole binds to thyroid peroxidase and thereby inhibits the conversion of iodide to iodine. Thyroid peroxidase normally converts iodide to iodine (via hydrogen peroxide as a cofactor) and also catalyzes the incorporation of the resulting iodide molecule onto both the three and/or five positions of the phenol rings of tyrosines found in thyroglobulin. Thyroglobulin is degraded to produce thyroxine (T4) and triiodothyronine (T3), which are the main hormones produced by the thyroid gland. So methimazole effectively inhibits the production of new thyroid hormones.'
The chemical IUPAC name of propylthiouracil is 6-propyl-2-sulfanylidene-1H-pyrimidin-4-one, its chemical formula is C7H10N2OS, its average molecular weight is 170.2321 g/mol, its melting point is 219 C, its experimental water solubility is 219 C, and its half life is two hours (DrugBank 2007c).
The chemical IUPAC name of carbimazole is ethyl 3-methyl-2-sulfanylideneimidazole-1-carboxylate, its chemical formula is C7H10N2O2S, its average molecular weight is 186.2315 g/mol, and its melting point is 123.5 C (DrugBank 2007d).
'Methimazole can be administered once per day. Methimazole may be initially prescribed at doses ranging from 10 to 40 milligrams per day. Once the hyperthyroidism is under control, doses of five to 15 milligrams per day are sufficient to control hyperthyroidism... PTU requires an average of 16.8 weeks to effectively lower T4 levels. PTU must be taken two to three times per the day, and several tablets are usually needed to achieve an effective dose. A dose of 300 to 600 milligrams per day may be recommended initially. Once the hyperthyroidism is under control, 100 to 200 milligrams per day are sufficient to control hyperthyroidism in most people.'
A major hazard of using the three medications (methimazole, propylthiouracil, and carbimazole) is an occasional suppression of the production of white blood cells by the bone marrow, which happens rarely to patients but patients are still advised to see a doctor when they develop a fever, sore throat, or other signs of infection (MedicineNet 2005b). 'The main shortcoming of antithyroid drugs is that the underlying hyperthyroidism often comes back after they are discontinued' (Endocrine Web 2005a). However, according to Ross (2008c) several factors such as antibodies to TSH receptors, individual factors, dose of antithyroid dru