Methimazole (MMI) and Propylthiouracil (PTU) Mechanism of Action

Propylthiouracil and methimazole are indicated for use in patients with hyperthyroidism. These drugs inhibit thyroid hormone synthesis, helping to alleviate symptoms in patients.

Propylthiouracil and Methimazole both work to block thyroid peroxidase, an enzyme which aids in the oxidation of iodide. This interrupts the organification of iodine, which leads to inhibition of thyroid hormone synthesis.

In contrast to methimazole, propylthiouracil has peripheral activity, where it inhibits the enzyme 5'-deiodinase, which converts T4 to its active form T3. By blocking this peripheral conversion, PTU helps alleviate symptoms of hyperthyroidism.

A rare side effect of these two medications is agranulocytosis, which is the decrease of white blood cells in the blood. Patients may become more prone to illness, fever and infectious lesions.

The most common side effect of these medications is skin rash, which can also include hives, pruritis, and pigmentation.

Propylthiouracil is unique as it can rarely cause fulminant liver failure at any course during treatment, which requires liver transplant.

Methimazole is linked with a teratogenic effect known as aplasia cutis, which is a congenital focal absence of epidermis. Pregnant patients should be placed on propylthiouracil instead of methimazole, especially in the first trimester of pregnancy.