Home / Trigger atlas / Medications & treatments / Methimazole & carbimazole (antithyroid drugs)
Medications · Updated 2026-10-30 · Editorial review in progress
Methimazole hair loss: the drug, or the overactive thyroid?
Also searched as: methimazole hair losscarbimazole hair lossGraves disease hair lossPTU hair loss
The British Thyroid Foundation says antithyroid drugs (carbimazole and propylthiouracil) can in rare cases cause diffuse hair loss, but in all probability the earlier overactivity is the cause, and it is unusual to need another treatment.
No consistent week-by-week window is published for this, so it is not on the calculator.
Sorting out the suspects
| Suspect | What the evidence says |
|---|---|
| Antithyroid drug | Rare cause of diffuse hair loss [British] |
| The overactive thyroid itself | Severe, prolonged hyperthyroidism causes diffuse loss, appearing months later [British] |
| Radioiodine | Does not cause hair loss [British] |
| Drug-related shedding in general | Usually evident 2–4 months after a change [DermNet] |
Methimazole is the active form of carbimazole. Antithyroid drugs appear on DermNet's list of medicines linked to telogen effluvium [DermNet].
Key facts
- It may be very difficult to tell whether the hair loss is due to previous overactivity or the antithyroid drug, and it is unusual to have to seek alternative treatment. [British]
Normal vs. warning signs
- Diffuse shedding a few months after diagnosis
- Gradual improvement as levels settle
- Patches or smooth bald spots
- Scalp pain, redness, burning or scaling
- Loss of eyebrows, eyelashes or body hair
- Sore throat, fever or mouth ulcers while on antithyroid drugs — contact your doctor urgently
What may help
- Never stop or change a prescribed medicine on your own — raise hair changes with your prescriber, who can weigh them against why you take it.
- Note the dates: diagnosis, starting the drug, dose changes, first shedding.
- Ask whether iron, ferritin or other tests make sense.
Related triggers
Untreated thyroid disease
MedicationsLevothyroxine changes
Hair signsTelogen club hair
Labs & explainersFerritin & iron
Frequently asked questions
Does methimazole cause hair loss?
Rarely. The British Thyroid Foundation says the overactive thyroid itself is the more probable cause.
Will my hair grow back after Graves' disease?
Regrowth is usual once the thyroid is successfully treated, though it takes several months and may be incomplete.
Does radioactive iodine make hair fall out?
No — BTF states radioiodine does not cause hair loss.
Sources
- British Thyroid Foundation. Hair loss and thyroid disorders
- DermNet. Alopecia from drugs