Drugs Affecting Thyroid Function

Drugs can have a significant impact on thyroid function, either by directly affecting thyroid hormone synthesis, secretion, and metabolism, or through interactions with the thyroid gland itself. Here's an overview of how some common drugs affect thyroid function:

1. Thyroid Hormone Synthesis Inhibitors:

• Amiodarone: This anti-arrhythmic drug contains high iodine content, which can induce both hyperthyroidism (excessive thyroid hormone) or hypothyroidism (reduced thyroid hormone).
• Lithium: Commonly used in psychiatric disorders, lithium inhibits the release of thyroid hormones and can lead to hypothyroidism.
• Thioamides (e.g., Methimazole, Propylthiouracil): These drugs treat hyperthyroidism by inhibiting thyroid hormone synthesis, specifically by blocking the thyroid peroxidase enzyme.

2. Drugs Altering Thyroid Hormone Metabolism:

• Glucocorticoids: High doses can inhibit the peripheral conversion of thyroxine (T4) to the more active triiodothyronine (T3), temporarily reducing thyroid hormone levels.
• Beta-Blockers (e.g., Propranolol): Used in managing symptoms of hyperthyroidism, these can also reduce peripheral conversion of T4 to T3.

3. Drugs Affecting Thyroid Hormone Absorption:

• Calcium and Iron Supplements: These can reduce the absorption of levothyroxine, a synthetic form of T4 used in hypothyroidism treatment.
• Proton Pump Inhibitors (PPIs) and Antacids: These can also interfere with levothyroxine absorption by altering stomach pH.

4. Drugs Causing Autoimmune Thyroid Disease:

• Interferons: Used in treating viral infections and some cancers, interferons can trigger autoimmune thyroiditis, leading to either hypothyroidism or hyperthyroidism.

5. Radiocontrast Agents:

• Iodinated Contrast Media: These can cause thyroid dysfunction, especially in individuals with underlying thyroid disease, by introducing large amounts of iodine that affect hormone synthesis.

Monitoring thyroid function is critical when administering these drugs, especially in patients with pre-existing thyroid disorders or at risk of thyroid dysfunction.