Carbimazole
A thionamide prodrug, rapidly converted to thiamazole, used to treat an overactive thyroid and to prepare for surgery or radioiodine.
Drug card
- Class
- Thionamide
- How it works
- Blocking thyroid peroxidase, so the thyroid makes less T4 and T3
- Used for
- Overactive thyroid (hyperthyroidism)
- Preparing for thyroid surgery or radioiodine
- Known for
- Agranulocytosis and other bone marrow suppression
- Birth defects (aplasia cutis, choanal atresia)
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does carbimazole work?
A prodrug that is almost completely converted to thiamazole (methimazole) soon after it is absorbed. Thiamazole is concentrated in the thyroid and blocks iodide organification and stops iodothyronine residues coupling, so less T4 and T3 is made. Stored hormone is unaffected, so the full effect takes 1 to 2 months.
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 carbimazole used for?
- Overactive thyroid (hyperthyroidism)
- Preparing for thyroid surgery or radioiodine
Side effects
Side effects of carbimazole
- Agranulocytosis and other bone marrow suppressionKnown forSerious
- Birth defects (aplasia cutis, choanal atresia)Known forSerious
- Jaundice, hepatitis and abnormal liver testsSerious
- Rash and itching
- Nausea and mild stomach upset
- Headache
- Joint pain
- Loss of taste
- Hair loss
- Acute pancreatitisSerious
- Cutaneous vasculitis and angioedemaSerious
- Low blood sugar from insulin autoimmune syndromeSerious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Sore throat, mouth ulcers, fever, bruising or malaise: stop it and check the white cell count at once.
- Contraindicated in serious blood disorders, severe liver failure, or after pancreatitis on carbimazole or thiamazole.
- Pregnancy: it crosses the placenta and can cause malformations, mostly in the first trimester; contraception is advised during use.
- It can strengthen warfarin, so the INR is checked; cross-allergy with thiamazole and propylthiouracil can occur.
Good to know: Not marketed in the US, where its active form, thiamazole (methimazole), is used instead. Side effects mostly appear in the first 8 weeks.
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:
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
Carbimazole is one of 9 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
Start Practicing