Overview
Propylthiouracil (PTU) is an antithyroid medication utilized for the management of hyperthyroidism. It is often reserved for patients who cannot tolerate methimazole or carbimazole, and is the preferred treatment option during the first trimester of pregnancy.

Mechanism of Action
Like other antithyroid drugs, PTU inhibits thyroid peroxidase, blocking the synthesis of thyroid hormones. Additionally, PTU has the unique property of inhibiting the peripheral conversion of T4 to the more active T3, making it particularly useful in the treatment of thyroid storm.

Dosage and Administration
The initial dose is generally 300 to 450 mg daily, divided into three equal doses given every 8 hours. Once a euthyroid state is achieved, the dose is tapered to a maintenance level of 100 to 150 mg daily.

Side Effects
Common adverse effects include skin rashes, itching, and joint pain. Serious risks include agranulocytosis and severe, potentially fatal hepatotoxicity. Due to the risk of liver failure, PTU is not recommended as a first-line therapy except in specific clinical scenarios.

Contraindications
It is contraindicated in patients with a history of severe hypersensitivity reactions or PTU-induced liver failure or agranulocytosis. Regular monitoring of liver function and complete blood counts is highly recommended during therapy.

Pregnancy and Lactation
PTU is the antithyroid drug of choice during the first trimester of pregnancy due to a lower risk of specific birth defects compared to methimazole/carbimazole. It is generally safe during breastfeeding, as only small amounts are excreted in breast milk.