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Pharmacology and toxicity of thionamides

Douglas S Ross, MD
Section Editor
David S Cooper, MD
Deputy Editor
Jean E Mulder, MD


Thionamide compounds were found in 1943 to inhibit thyroid hormone synthesis. They are actively transported into the thyroid gland where they inhibit both the organification of iodine to tyrosine residues in thyroglobulin and the coupling of iodotyrosines (figure 1) [1].

The pharmacology and toxicity of propylthiouracil (PTU) and methimazole will be reviewed here. Specific issues related to their use in the treatment of Graves' disease are discussed separately. (See "Thionamides in the treatment of Graves' disease".)


Propylthiouracil (PTU) and methimazole are the thionamide drugs available in the United States. Carbimazole is a third option available in some European and Asian countries. Methimazole and carbimazole are interchangeable because carbimazole is completely metabolized to methimazole. The carbimazole dose required to yield an equivalent dose of methimazole is approximately 40 percent higher.

PTU, but not methimazole, inhibits the 5'-monodeiodinase that converts thyroxine (T4) to triiodothyronine (T3) in extrathyroidal tissue. However, methimazole has several important pharmacokinetic advantages over PTU:

The serum half-life of methimazole is four to six hours, whereas that of PTU is 75 minutes.

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Literature review current through: Nov 2017. | This topic last updated: Oct 17, 2016.
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