Propylthiouracil (PTU)

Propylthiouracil (PTU) is an antithyroid drug that reduces thyroid hormone production by blocking thyroid peroxidase. It also reduces the conversion of T4 to T3 in peripheral tissues, which can be useful in severe hyperthyroidism or thyroid storm.

In the UK, PTU is generally reserved for specific situations rather than first-line use, because of a small risk of severe liver toxicity (hepatotoxicity) - rare but more common with PTU than carbimazole.

The main use of PTU in the UK is during the first trimester of pregnancy: carbimazole carries a small risk of birth defects in early pregnancy, while PTU does not share the same risk. Guidelines therefore recommend PTU in the first trimester, with the option to switch back to carbimazole from the second trimester onwards.

As with carbimazole, PTU can rarely cause agranulocytosis (low white blood cells). Anyone taking PTU who develops a sore throat, fever, or signs of infection should seek urgent medical assessment. Any suggestion of liver problems - jaundice, dark urine, or abdominal pain - should also prompt stopping PTU and urgent review.