Propylthiouracil

An antithyroid medication used to treat hyperthyroidism

Propylthiouracil (PTU) is an antithyroid drug used primarily to manage hyperthyroidism, a condition characterized by excessive production of thyroid hormones by the thyroid gland. It is particularly used in cases of Graves' disease, a common cause of hyperthyroidism.

Mechanism of Action

Propylthiouracil works by inhibiting the enzyme thyroid peroxidase, which plays a crucial role in the synthesis of thyroid hormones. This enzyme catalyzes the iodination of tyrosine residues in thyroglobulin and the coupling of iodotyrosines to form triiodothyronine (T3) and thyroxine (T4). By blocking this enzyme, PTU reduces the production of T3 and T4, thereby decreasing the levels of circulating thyroid hormones.

Diagram of thyroid hormone synthesis, showing the role of thyroid peroxidase.

Additionally, propylthiouracil inhibits the peripheral conversion of T4 to the more active T3, further contributing to its antithyroid effects.

Indications

Propylthiouracil is indicated for:

• Treatment of hyperthyroidism due to Graves' disease.
• Preparation of patients for thyroidectomy or radioactive iodine therapy.
• Management of thyroid storm, a life-threatening exacerbation of hyperthyroidism.

Dosage and Administration

The dosage of propylthiouracil varies depending on the severity of the hyperthyroidism and the patient's response to treatment. It is typically administered orally, with doses ranging from 100 mg to 600 mg per day, divided into multiple doses.

Side Effects

Common side effects of propylthiouracil include:

• Rash
• Nausea
• Arthralgia

Serious side effects, though rare, can occur and include:

• Agranulocytosis, a potentially life-threatening decrease in white blood cells.
• Hepatotoxicity, which can lead to liver failure.

Contraindications

Propylthiouracil is contraindicated in patients with known hypersensitivity to the drug. Caution is advised in patients with pre-existing liver disease.

Pregnancy and Lactation

Propylthiouracil is preferred over methimazole during the first trimester of pregnancy due to the lower risk of teratogenic effects. However, it should be used with caution and under close medical supervision.

Also see

• Hyperthyroidism
• Graves' disease
• Thyroid hormone
• Methimazole
• Thyroidectomy

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