💊 Propylthiouracil

Propylthiouracil (PTU) is an antithyroid drug, belonging to the thionamide class, used to treat hyperthyroidism.

PTU inhibits the enzyme thyroid peroxidase, blocking the organification of iodide and the coupling of iodotyrosines, thereby preventing the synthesis of thyroid hormones (T3 and T4). Uniquely, it also inhibits the peripheral conversion of T4 to the more active T3.

Primarily indicated for the treatment of hyperthyroidism, particularly in patients who cannot tolerate carbimazole or methimazole. It is the preferred antithyroid drug in the first trimester of pregnancy due to a lower risk of teratogenicity compared to carbimazole, and in thyroid storm.

Known hypersensitivity to PTU or other thionamides. Severe hepatic impairment or a history of PTU-induced hepatotoxicity are absolute contraindications. Pre-existing bone marrow depression or a history of PTU-induced agranulocytosis are also contraindications. Pregnancy in later trimesters may favour carbimazole.

Common adverse effects include rash, pruritus, arthralgia, and nausea. The most serious adverse effect is severe hepatotoxicity, which can be fatal, and agranulocytosis. Other serious effects include vasculitis and aplastic anaemia.

Full blood count (FBC) should be performed before treatment and patients must be advised to report any signs of infection (e.g., fever, sore throat) immediately for an urgent FBC. Liver function tests (LFTs) are crucial and should be monitored frequently, especially in the initial months. Thyroid function tests (TSH, free T4, free T3) are monitored to assess treatment efficacy.

Counsel patients extensively on the symptoms of agranulocytosis (fever, sore throat) and hepatotoxicity (jaundice, dark urine, abdominal pain) and the need for urgent medical review. Always check baseline FBC and LFTs, and monitor LFTs closely. Discuss its use in pregnancy, highlighting its preference in the first trimester.

High-yield for its specific role in the first trimester of pregnancy and in thyroid storm. Remember its unique additional mechanism of inhibiting peripheral T4 to T3 conversion. Be aware of the significant risk of hepatotoxicity, which is more pronounced than with carbimazole. Agranulocytosis risk is similar to carbimazole.

SBAs often test the specific indication for PTU in the first trimester of pregnancy or in thyroid storm. Questions may also focus on its unique mechanism of inhibiting peripheral T4 to T3 conversion, or its serious adverse effect of hepatotoxicity. Expect scenarios involving a pregnant hyperthyroid patient or a patient presenting with thyroid storm.