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Antithyroid Drug – Mechanism, Uses & Side Effects

An antithyroid drug is a medication that reduces the excessive production of thyroid hormones. It is primarily used to treat hyperthyroidism (overactive thyroid gland).

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Things worth knowing about "Antithyroid Drug"

An antithyroid drug is a medication that reduces the excessive production of thyroid hormones. It is primarily used to treat hyperthyroidism (overactive thyroid gland).

What Is an Antithyroid Drug?

An antithyroid drug (also called a thyreostaticum or thyrostatic agent) is a medication that selectively inhibits the synthesis of thyroid hormones – primarily thyroxine (T4) and triiodothyronine (T3) – in the thyroid gland. These drugs are mainly used to treat hyperthyroidism, a condition in which the thyroid gland produces too many hormones, accelerating a wide range of body functions.

Indications

Antithyroid drugs are indicated in the following conditions and situations:

Graves disease: An autoimmune disorder and the most common cause of hyperthyroidism
Toxic adenoma: A benign thyroid nodule that autonomously produces hormones
Toxic multinodular goiter: Multiple hormone-producing nodules within the thyroid gland
Preparation for thyroid surgery or radioiodine therapy
Thyrotoxic crisis (thyroid storm): A life-threatening escalation of thyroid overactivity (emergency treatment)
Management of hyperthyroidism during pregnancy (under strict medical supervision)

Mechanism of Action

Antithyroid drugs interfere with the biosynthesis of thyroid hormones. The main active substances are thiamazole, carbimazole (a prodrug of thiamazole), and propylthiouracil (PTU). They inhibit the enzyme thyroid peroxidase (TPO), which is essential for the iodination of tyrosine residues on thyroglobulin – a key step in hormone synthesis.

Propylthiouracil also inhibits the peripheral conversion of T4 into the more biologically active T3, which can be particularly beneficial in managing severe hyperthyroidism or thyroid storm.

Because the thyroid gland stores a reserve of preformed hormones, the full therapeutic effect of antithyroid drugs typically takes several weeks to become apparent.

Dosage and Administration

Dosing is individualized based on the severity of hyperthyroidism and the patient's response to treatment. Typical dosage guidance includes:

Thiamazole: Initial dose commonly 10–40 mg per day; maintenance dose 2.5–10 mg per day
Carbimazole: Initial dose commonly 15–60 mg per day; converted to thiamazole in the body
Propylthiouracil: Initial dose commonly 200–400 mg per day in divided doses; preferred in the first trimester of pregnancy

Treatment duration is generally 12–18 months, particularly for Graves disease. Regular monitoring of thyroid function tests (TSH, T3, T4) is required throughout therapy.

Side Effects

Antithyroid drugs are generally well tolerated, but may cause the following side effects:

Common: Skin rash, itching, joint pain, mild elevation of liver enzymes
Rare but serious: Agranulocytosis (a severe drop in white blood cell count with increased infection risk) – patients should seek immediate medical attention if they develop fever or sore throat
Rare: Liver damage (especially with propylthiouracil), vasculitis (inflammation of blood vessels)
Hypothyroidism (underactive thyroid) if the dose is too high

Patients on antithyroid therapy should contact a healthcare provider promptly if they experience unusual symptoms – particularly sore throat, fever, or jaundice.

Treatment Context and Alternatives

Antithyroid drugs are often the first-line treatment option for hyperthyroidism. Depending on the underlying cause, severity, and individual patient profile, the following therapies may also be considered:

Radioiodine therapy: Radioactive iodine selectively destroys thyroid tissue
Thyroid surgery (thyroidectomy): Partial or total removal of the thyroid gland

After a permanent reduction of thyroid function through radioiodine or surgery, lifelong hormone replacement therapy with levothyroxine is often required.

References

European Thyroid Association (ETA): Guidelines for the Management of Graves' Hyperthyroidism. 2022.
Ross DS et al. – 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism. Thyroid. 2016;26(10):1343–1421.
Herold G. et al. – Internal Medicine (Innere Medizin). Cologne, 2023.

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