Thionamides

They inhibit thyroid peroxidase, the enzyme that attaches iodine to tyrosine residues in thyroglobulin and couples them into T4 and T3, so less new hormone is made. Hormone already stored or circulating is not touched, so the full effect takes weeks. Propylthiouracil also blocks the conversion of T4 to T3 in the tissues.

• CarbimazoleNot marketed in the US, where its active form, thiamazole (methimazole), is used instead. Side effects mostly appear in the first 8 weeks.
• ThiamazolePreferred over propylthiouracil outside early pregnancy, because its risk of severe liver injury is lower, especially in children.
• PropylthiouracilIt also blocks T4-to-T3 conversion in the tissues, but its boxed warning for liver failure keeps it for early pregnancy, thyroid storm and thiamazole intolerance.

• Overactive thyroid (Graves' disease, toxic goiter)
• Preparing for thyroid surgery or radioiodine
• Thyroid storm (propylthiouracil)

• AgranulocytosisKnown forSerious
• Liver injury, worst with propylthiouracilKnown forSerious
• Rash and itching
• Underactive thyroid from over-treatment
• Birth defects (carbimazole, thiamazole in early pregnancy)Serious
• ANCA-associated vasculitisSerious
• Joint pain
• Nausea and stomach upset
• Loss of taste
• Acute pancreatitis (carbimazole, thiamazole)Serious

What the class has in common. Each drug's card has its own, and Known for marks what an exam expects you to know.

• Sore throat, fever, mouth ulcers or other signs of infection: stop the drug and check the white cell count at once.
• Pregnancy: propylthiouracil may be preferred in the first trimester, and carbimazole or thiamazole after it.
• Anorexia, itching or right upper abdominal pain suggest liver injury: check liver tests promptly and stop the drug if confirmed.
• They pass to the fetus and may cause a fetal goiter, so the lowest dose that controls the mother's disease is used.

Written for biofideX in plain words, after checking these references:

• Methimazole tablets: label (DailyMed)
• Propylthiouracil tablets: label (DailyMed)
• Carbimazole 10mg Tablets: SmPC (emc)
• Antithyroid Agent (NCI Thesaurus)
• Carbimazole (NHS)

For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.