Antithyroid drugs (carbimazole, methimazole, propylthiouracil)

What it is

Drugs that block thyroid hormone production in hyperthyroidism such as Graves disease. Methimazole and carbimazole are first line; propylthiouracil is reserved for specific situations. All are prescription-only. Radioiodine and surgery are alternatives that can lead to an underactive thyroid, which also affects hair.

How it works

Thyroid dysfunction, overactive, underactive or drug-induced, can cause widespread shedding. The incidence in hair patients is poorly known. Hussein 2023

What the studies show

• Of 500 women with telogen effluvium, 248 had normal thyroid function, 150 were hypothyroid and 102 hyperthyroid. The hypothyroid group had the most severe loss; the hyperthyroid group did not differ from normal. Bin Dayel 2024
• In a chart review of 1,411 patients with alopecia areata, telogen effluvium or pattern loss, TSH did not differ across types in people with normal thyroid function. Thyroid dysfunction was found in 8.5%, most often in telogen effluvium. Alomary 2026
• A meta-analysis of 17 studies found thyroid disease more common in alopecia areata (OR 3.66). That is autoimmune hair loss, separate from shedding. Xin 2020
• The US methimazole label lists abnormal loss of hair as a minor adverse reaction, and the propylthiouracil label lists it too, without frequencies. Methimazole label PTU label

No controlled trial compared hair outcomes on these drugs, and incidence is unknown. Shedding that starts 2 to 4 months after a major hormone change fits ordinary telogen effluvium timing. Tosi 1994

Side effects and safety

• Methimazole can cause a potentially life-threatening drop in white blood cells; report fever or sore throat immediately.
• Propylthiouracil carries a boxed warning for severe liver injury.
• Methimazole can harm a fetus in the first trimester, so anyone planning pregnancy needs specialist guidance.
• High-dose biotin can distort thyroid blood tests; see biotin.

Bottom line

If thyroid disease is the cause, treating it is the hair treatment, and hair often recovers over months once levels are stable. New shedding on an antithyroid drug deserves a thyroid-function check before blaming the drug, and an iron, vitamin D and B12 check as usual (iron and ferritin, zinc). Do not stop the drug to protect hair. Discuss with a doctor.