Antithyroid Drugs: Mechanism, Therapeutic Use, and Adverse Effects

Mechanism of Action

• Thionamides inhibit thyroid peroxidase, preventing the iodination of tyrosine residues in thyroglobulin and thus reducing T3 and T4 synthesis.
• Propylthiouracil (PTU) additionally inhibits the conversion of T4 to T3 in peripheral tissues.

Therapeutic Use

• Used to treat hyperthyroidism in conditions like Graves' disease, toxic multinodular goiter, and thyrotoxicosis.
• Preoperative preparation for thyroidectomy or radioactive iodine therapy.
• Management of thyroid storm.

Adverse Effects

• Rash
• Agranulocytosis (decreased white blood cells)
• Hepatotoxicity (liver damage), especially with PTU
• GI upset (nausea, vomiting)
• Arthralgia (joint pain)
• Goiter (enlarged thyroid gland)

Contraindications

• Pregnancy: Methimazole is contraindicated in the first trimester; PTU is preferred but should only be used if necessary.
• Liver disease: Avoid PTU due to hepatotoxicity risk.

Mechanism of Action

• Iodine compounds inhibit thyroid hormone release and prevent the iodination of tyrosine residues in thyroglobulin, reducing thyroid hormone synthesis.
• They also reduce the vascularity of the thyroid gland.

Therapeutic Use

• Used in thyroid storm to quickly reduce thyroid hormone secretion.
• Preoperative use to reduce the size and vascularity of the thyroid gland before surgery.
• Used in radiation emergencies to prevent thyroid damage from radioactive iodine exposure.

Adverse Effects

• Iodism: Symptoms like a metallic taste, burning sensation in the mouth, and skin rashes.
• Hypersensitivity reactions: Swelling, redness, and irritation.
• Goiter with prolonged use.
• Acneiform rash.

Contraindications

• Pregnancy: Caution advised, generally avoided due to risk to the fetus.
• Hypersensitivity to iodine: Contraindicated in patients with a history of iodine allergy.

Thionamides and iodine compounds are essential for managing hyperthyroidism. Thionamides inhibit thyroid hormone synthesis and conversion, while iodine compounds inhibit the release of hormones. Both classes have specific uses, adverse effects, and contraindications that must be considered in clinical practice.