Propylthiouracil (PTU): when it is used, doses and the liver warning

Propylthiouracil, almost always shortened to PTU, is the third antithyroid tablet alongside carbimazole (Neo-Mercazole) and methimazole. It works, and it has been used for decades, but rare severe liver injury has pushed it to second place. It keeps one important job: treating an overactive thyroid in the first three months of pregnancy.

Key points

• When it is used: if methimazole or carbimazole cannot be tolerated, in the first trimester of pregnancy, and sometimes in thyroid storm.
• Tablet: 50 mg. Usual start 300 mg a day in three doses; maintenance 100 to 150 mg a day.
• Boxed warning: severe liver injury and liver failure, sometimes fatal, in adults and children.
• Not for children unless nothing else is suitable.
• Equivalence: 50 mg of PTU is treated as equal to 5 mg of carbimazole.

Why PTU is second choice

The US label opens with a boxed warning: severe liver injury and acute liver failure, in some cases fatal or needing a liver transplant, have been reported in adults and children. The label reserves PTU for people who cannot tolerate methimazole and for whom radioactive iodine or surgery are not suitable. It adds that routine liver blood tests are not expected to prevent this injury, because it starts quickly and unpredictably.

The evidence points the same way. An analysis of more than 40 years of US adverse event reports found severe liver injury reported far more often than expected with PTU in under-17s, but not with methimazole. And in a Japanese randomised trial, methimazole 30 mg a day normalised hormone levels in 96.5% of patients by 12 weeks against 78.3% on PTU 300 mg, with the authors advising against PTU as a first choice.

When PTU is still the right drug

Early pregnancy. Carbimazole and methimazole are linked with a pattern of birth defects after first-trimester use. The European Thyroid Association advises switching to PTU when planning a pregnancy and during the first trimester. PTU is not free of risk either: in a Danish study of 817,093 births, birth defects were recorded in 8.0% of children exposed to PTU early in pregnancy and 9.1% after methimazole or carbimazole, against 5.7% in unexposed children, but the types of defect differed. The US label suggests considering a switch away from PTU after the first trimester because of its liver risk.

Thyroid storm. Besides blocking hormone production, PTU stops the body converting T4 into the more active T3. The label notes this may make it an effective treatment for thyroid storm, a hospital emergency.

Intolerance. Someone who had a minor reaction to methimazole or carbimazole may be offered PTU. After a serious reaction such as agranulocytosis, cross-reaction risk means another antithyroid drug is usually avoided.

Dose

PTU is short-acting, so the daily dose is split into three, about eight hours apart.

When switching between drugs, the Neo-Mercazole product information treats one 50 mg PTU tablet as equal to one 5 mg carbimazole tablet. Your doctor will check thyroid blood tests a few weeks after any change.

Side effects and warnings

• Liver: the boxed warning above. The label asks patients to watch especially closely in the first six months.
• Agranulocytosis: about 0.2% to 0.5% of patients, usually within the first three months.
• Vasculitis: mostly ANCA-positive, affecting the kidneys, skin, lungs, brain or bowel; some cases were fatal.
• Underactive thyroid: too high a dose; TSH and free T4 guide adjustments.
• Bleeding: PTU can lower clotting factors, so prothrombin time may be checked, especially before surgery.

Breastfeeding

Very little PTU reaches breast milk: in one study of nine women given 400 mg, about 0.025% of the dose appeared in milk over four hours. The label describes the amount the baby receives as likely clinically insignificant.

Interactions

As with methimazole and carbimazole: warfarin may work more strongly, and beta blockers, digoxin and theophylline may need lower doses once the thyroid is back to normal.

Where to get it

PTU is prescription only, and none of our main partner pharmacies list it, so fill it locally. If your doctor moves you on to the first-line drugs, methimazole is listed by PockDose and thiamazole by Yasabai, and carbimazole as Neo-Mercazole by VGR Australia. Read more in Neo-Mercazole compared and methimazole.

Sources

Checked 15 October 2026.

1. Propylthiouracil tablets prescribing information and Medication Guide (Rev. 7/2023) — DailyMed, US National Library of Medicine
2. Neo-Mercazole New Zealand data sheet (dose equivalence with thiouracils) — Medsafe (New Zealand)
3. 2018 European Thyroid Association guideline for the management of Graves' hyperthyroidism — European Thyroid Journal
4. Comparison of methimazole and propylthiouracil in Graves' disease (Nakamura et al., 2007) — Journal of Clinical Endocrinology & Metabolism
5. Dissimilar hepatotoxicity profiles of propylthiouracil and methimazole in children (Rivkees and Szarfman, 2010) — Journal of Clinical Endocrinology & Metabolism
6. Birth defects after early pregnancy use of antithyroid drugs, a Danish nationwide study (Andersen et al., 2013) — Journal of Clinical Endocrinology & Metabolism