Methimazole is preferable to propylthiouracil (except in pregnancy and thyroid storm) because it has a lower risk of serious liver injury and can be administered once daily, which may improve adherence. Antithyroid drug therapy is usually begun with divided doses, shifting to maintenance therapy with single daily doses when the patient becomes clinically euthyroid. However, mild to moderately severe thyrotoxicosis can often be controlled with methimazole given in a single morning dose of 20–40 mg initially for 4–8 weeks to normalize hormone levels. Maintenance therapy requires 5–15 mg once daily. Alternatively, therapy is started with propylthiouracil, 100–150 mg every 6 or 8 hours until the patient is euthyroid, followed by gradual reduction of the dose to the maintenance level of 50–150 mg once daily. In addition to inhibiting iodine organification, propylthiouracil also inhibits the conversion of T4 to T3, so it brings the level of activated thyroid hormone down more quickly than does methimazole. The best clinical guide to remission is reduction in the size of the goiter. Laboratory tests most useful in monitoring the course of therapy are serum FT3, FT4, and TSH levels.
Reactions to antithyroid drugs have been described above. A minor rash can often be controlled by antihistamine therapy. Because the more severe reaction of agranulocytosis is often heralded by sore throat or high fever, patients receiving antithyroid drugs must be instructed to discontinue the drug and seek immediate medical attention if these symptoms develop. White cell and differential counts and a throat culture are indicated in such cases, followed by appropriate antibiotic therapy. Treatment should also be stopped if significant elevations in transaminases (two to three times the upper limit of normal) occur.
B. Thyroidectomy
A near-total thyroidectomy is the treatment of choice for patients with very large glands or multinodular goiters. Patients are treated with antithyroid drugs until euthyroid (about 6 weeks). In addition, for 10–14 days prior to surgery, they receive saturated solution of potassium iodide, 5 drops twice daily, to diminish vascularity of the gland and simplify surgery. About 80–90% of patients will require thyroid supplementation following near-total thyroidectomy.
C. Radioactive Iodine
Radioiodine therapy (RAI) utilizing 131I is the preferred treatment for most patients over 21 years of age. In patients without heart disease, the therapeutic dose may be given immediately in a range of 80–120 μCi/g of estimated thyroid weight corrected for uptake. In patients with underlying heart disease or severe thyrotoxicosis and in elderly patients, it is desirable to treat with antithyroid drugs (preferably methimazole) until the patient is euthyroid. The medication is stopped for 2 to 3 days before RAI is administered so as not to interfere with RAI retention but can be restarted 3–5 days later, and then gradually tapered over 4–6 weeks as thyroid function normalizes. Iodides should be avoided to ensure maximal 131I uptake. Six to 12 weeks following the administration of RAI, the gland will shrink in size and the patient will usually become euthyroid or hypothyroid. A second dose may be required if there is minimal response 3 months post-RAI. Hypothyroidism occurs in about 80% of patients following RAI. Serum FT4 and TSH levels should be monitored regularly. When hypothyroidism develops, prompt replacement with oral levothyroxine, 50–150 mcg daily, should be instituted.
D. Adjuncts to Antithyroid Therapy