Therapeutic Uses : Anti Thyroid Drugs

The antithyroid drugs are used in the treatment of hyperthyroidism in the following three ways: (1) as definitive treatment, to control the disorder in anticipation of a spontaneous remission in Graves' disease; (2) in conjunction with radioactive iodine, to hasten recovery while awaiting the effects of radiation; and (3) to control the disorder in preparation for surgical treatment.

The usual starting dose for propylthiouracil is 100 mg every 8 hours or 150 mg every 12 hours. When doses larger than 300 mg daily are needed, further subdivision of the time of administration to every 4 to 6 hours is occasionally helpful. Methimazole is effective when given as a single daily dose because of its relatively long plasma and intrathyroidal half-life, as well as its long duration of action. Failures of response to daily treatment with 300 to 400 mg of propylthiouracil or 30 to 40 mg of methimazole are most commonly due to noncompliance. Delayed responses also are noted in patients with very large goiters or those in whom iodine in any form has been given beforehand. Once euthyroidism is achieved, usually within 12 weeks, the dose of antithyroid drug can be reduced.

After treatment is initiated, patients should be examined and thyroid function tests (serum free thyroxine index and total triiodothyronine concentrations) measured every 2 to 4 months. Once euthyroidism is established, follow-up every 4 to 6 months is reasonable.

Thyrotoxicosis in Pregnancy- Thyrotoxicosis occurs in about 0.2% of pregnancies and is caused most frequently by Graves' disease. Antithyroid drugs are the treatment of choice; radioactive iodine is clearly contraindicated. Propylthiouracil is preferred over methimazole because of its lower transplacental passage. Propylthiouracil is the drug of choice in nursing women; the very small amounts of the drug that appear in breast milk do not appear to affect thyroid function in the suckling baby.

Adjuvant Therapy-Several drugs that have no intrinsic antithyroid activity are useful in the symptomatic treatment of thyrotoxicosis. -Adrenergic receptor antagonists are effective in antagonizing the catecholaminergic effects of thyrotoxicosis by reducing the tachycardia, tremor, and stare and relieving palpitations, anxiety, and tension. Either propranolol, 20 to 40 mg four times daily, or atenolol, 50 to 100 mg daily, is usually given initially.

Preoperative Preparation- Patients must be rendered euthyroid prior to subtotal thyroidectomy as definitive treatment for hyperthyroidism to reduce operative morbidity and mortality. It is possible to bring virtually 100% of patients to a euthyroid state; the operative mortality in these patients in the hands of an experienced thyroid surgeon is extremely low. Prior treatment with antithyroid drugs usually is successful in rendering the patient euthyroid for surgery. Iodide is added to the regimen for 7 to 10 days prior to surgery to decrease the vascularity of the gland, making it less friable and decreasing the difficulties for the surgeon. In the patient who is either allergic to antithyroid drugs or is noncompliant, a euthyroid state usually can be achieved by treatment with iopanoic acid, dexamethasone, and propranolol for 5 to 7 days prior to surgery. All of these drugs should be discontinued after surgery.

Category: Pharmacology Notes