Thiamazole
A thionamide that blocks thyroid peroxidase; the preferred antithyroid drug for Graves' disease and toxic goiter outside early pregnancy.
Also called methimazole (US name).
Drug card
- Class
- Thionamide
- How it works
- Blocking thyroid peroxidase, so the thyroid makes less T4 and T3
- Used for
- Graves' disease and toxic multinodular goiter
- Preparing for thyroid surgery or radioiodine
- Known for
- Agranulocytosis
- Birth defects with first-trimester use
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does thiamazole work?
A thioamide that inhibits thyroid peroxidase, so iodide is not used to iodinate the tyrosine residues of thyroglobulin and T4 and T3 synthesis falls. It does not inactivate hormone already stored in the gland or circulating, so control comes gradually. It is also the active form of carbimazole.
Like other 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.
Uses
What is thiamazole used for?
- Graves' disease and toxic multinodular goiter
- Preparing for thyroid surgery or radioiodine
Side effects
Side effects of thiamazole
- AgranulocytosisKnown forSerious
- Birth defects with first-trimester useKnown forSerious
- Hepatitis, rarely acute liver failureSerious
- Underactive thyroid from over-treatment
- ANCA-associated vasculitisSerious
- Low blood sugar from insulin autoimmune syndromeSerious
- Acute pancreatitisSerious
- Rash and hives
- Nausea and vomiting
- Joint pain
- Loss of taste
- Hair loss
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Fever or sore throat: check for agranulocytosis at once and stop the drug if it is confirmed.
- First trimester: linked to aplasia cutis and other malformations, so another drug may be better in early pregnancy.
- Liver symptoms call for liver tests; stop it if transaminases rise above three times normal.
- It can increase warfarin's effect; as the thyroid settles, beta blocker, digoxin and theophylline doses may need lowering.
Good to know: Preferred over propylthiouracil outside early pregnancy, because its risk of severe liver injury is lower, especially in children.
Remember it
A way to remember it
PTU for the first trimester and thyroid storm; carbimazole or thiamazole for everyone else, and any sore throat means a blood count.
Propylthiouracil carries a boxed warning for liver failure, so it is kept for early pregnancy (when carbimazole and thiamazole are linked to birth defects) and thyroid storm, where its block on T4-to-T3 conversion helps. All three can cause agranulocytosis.
Written for biofideX
Its class
Thionamides
Other thionamides with a card:
Easily mixed up with thyroid hormones.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Methimazole tablets: label (DailyMed)
- Methimazole (NCI Thesaurus)
- Carbimazole 10mg Tablets: SmPC (emc)
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.
Thyroid, steroids and bones
Thiamazole is one of 9 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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