Pharmacology of Thyroid Medications Quiz

Pharmacology of Levothyroxine, Radioiodine, and Thioamide Drugs

Levothyroxine, radioiodine, and thioamides are essential medications used primarily in the treatment of hypothyroidism, hyperthyroidism, goiter, and thyroid cancer. These drugs work through various mechanisms, have distinct indications, and can cause different side effects.

Indications and What They Treat

Levothyroxine

FDA-approved uses of levoothyroxine include hypothyroidism caused by Hashimoto's disease, lymphoma, partial thyroidectomy, and radiation therapy for thyroid cancer; myxedema coma; prevention of hypothyroidism in high-risk patients with thyroid cancer.

Radioiodine

Radioactive iodine is indicated for the treatment of well-differentiated thyroid carcinomas, such as papillary and follicular tumors. It is also used for treating both medullary thyroid carcinomas and thyroid carcinomas of unknown histological type.

Thioamides

Thyroid hormone inhibitors (thioamides) are mainly used for the treatment of hyperthyroidism associated with Graves' disease. However, their primary indication remains the management of toxic multinodular goiter, especially if it is refractory to other treatments like beta blockers or antithyroid drugs.

Mechanism of Action

Levothyroxine

As a synthetic form of triiodothyronine (T3) and thyroxine (T4), levothyroxine increases levels of circulating T3 and T4 in the body. It supplies thyroxine when the body cannot produce enough of its own, either because of an underactive thyroid gland or because the existing thyroid tissue has been removed surgically.

Radioiodine

Radioiodines emit ionizing radiation that destroys the thyroid cells involved in synthesizing thyroid hormones and reduces the overall production of these hormones. This results in reduced secretion of thyroid hormones from the remaining thyroid tissue.

Thioamides

Propylthiouracil and methylazole are two common thioamide drugs that act by blocking the production of thyroid hormones. They inhibit the enzyme thyroperoxidase, which is necessary for the formation of thyroglobulin—the building block for thyroid hormones—and thus reduce the production of T3 and T4 hormones.

Potential Side Effects

Levothyroxine

The most commonly reported side effects of levothyroxine are headache and gastrointestinal disturbances like nausea, vomiting, and abdominal pain. Other possible side effects include chest pain, tremors, and weight loss.

Radioiodine

The most important side effect of radioactive iodine therapy is the risk of secondary malignancies, which is significantly increased in pediatric patients with papillary thyroid cancer, compared to individuals treated with surgery alone. Other side effects may include temporary hypothyroidism, a condition known as post-ablative hypothyroidism.

Thioamides

The most severe side effects of thioamides include agranulocytosis, which is rare but can be life-threatening, and disturbances in liver function tests. Other potential side effects include nausea, abdominal pain, rash, and pruritus.