Pharmacology and toxicity of thionamides
- Douglas S Ross, MD
Douglas S Ross, MD
- Section Editor — Thyroid Disease
- Professor of Medicine
- Harvard Medical School
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) .
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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- TOXICITIES AND THEIR MANAGEMENT
- Common, minor side effects
- Potential teratogenicity
- Rare but serious adverse effects
- - Agranulocytosis
- - Hepatotoxicity
- - ANCA-positive vasculitis
- - Pancreatitis
- - Insulin autoimmune syndrome
- INFORMATION FOR PATIENTS
- SUMMARY AND RECOMMENDATIONS