propylthiouracil (cont.)

DOSING: The initial adult dose of PTU is 300 mg/day divided into 3 divided doses. PTU is usually administered every eight hours. Patients with large goiters may require initial doses up to 900 mg daily. The usual long-term adult dose after initial treatment is 100-150 mg/day.

DRUG INTERACTIONS: PTU may increase the effect of oral blood thinners, for example warfarin (Coumadin). Therefore, warfarin dose changes and monitoring for the effects of warfarin on blood clotting are necessary.

Hyperthyroidism may cause increased elimination of beta blockers (for example, propranolol [Inderal, Inderal LA, Innopran XL)). Once hyperthyroidism is reversed the excretion of beta-blockers may return to normal and less beta-blocker will be needed to provide the same effect.

Digoxin (Lanoxin) blood levels may be increased when hyperthyroidism is reversed in patients on a stable digoxin dose. A smaller dose of digoxin may be needed in order to avoid toxicity of digoxin. Similarly, theophylline elimination may decrease when hyperthyroidism is reversed in patients on a stable theophylline dose. A reduced dose of theophylline may be needed in order to avoid toxicity of theophylline.

PREGNANCY: PTU crosses the placenta and may cause harm to the fetus. If it is necessary to use PTU during pregnancy the lowest effective dose should be used. Since methimazole is associated with fetal abnormalities, PTU is used during the first trimester if an antithyroid drug is needed.

NURSING MOTHERS: PTU is excreted in breast milk in small amounts.

SIDE EFFECTS: The most common side effects are related to the skin and include rash, itching, hives, abnormal hair loss, and skin pigmentation. Other common side effects are swelling, nausea, vomiting, heartburn, loss of taste, joint or muscle aches, numbness and headache. Less common but serious side effects have occurred with PTU therapy. A decrease of white blood cells in the blood (agranulocytosis) may occur. Symptoms and signs of agranulocytosis include infectious lesions of the throat, the gastrointestinal tract, and skin with an overall feeling of illness and fever.

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