Primary hypothyroidism is usually the result of Hashimoto's thyroiditis, thyroidectomy for hyperthyroidism, goiter or cancer, or radioactive iodine therapy for hyperthyroidism.

The thionamides propylthiouracil and methimazole are both used to treat hyperthyroidism.1 Propylthiouracil causes severe hepatic toxicity or hepatic failure in about 0.1% of adults and children.2,3 It is the third leading cause of liver transplants due to drug toxicity (acetaminophen and isoniazid are the first two). Methimazole may cause less serious hepatic toxicity; reversible cholestatic jaundice has been reported. There is generally no good reason to continue to use propylthiouracil, with 2 possible exceptions. First, propylthiouracil may be preferred for treatment of life-threatening thyroid storm because it inhibits conversion of T4 to T3, while methimazole does not. Second, propylthiouracil may be preferable for use in pregnancy because use of methimazole has rarely been associated with aplasia cutis and with choanal and esophageal atresia.

1. Drugs for thyroid disorders. Treat Guidel Med Lett 2009; 7:57.
2. SA Rivkees and DR Mattison. Ending propylthiouracil-induced liver failure in children. N Engl J Med 2009; 360:1574.
3. DS Cooper and SA Rivkees. Putting propylthiouracil in perspective. J Clin Endocrinol Metab 2009; 94:1881.

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Hypothyroidism is usually the result of Hashimoto's thyroiditis, thyroidectomy for hyperthyroidism or cancer, or radioactive iodine therapy. Treatment of hypothyroidism with replacement doses of thyroid hormone is usually lifelong. The drug of choice for thyroid replacement therapy is levothyroxine. Liothyronine and dessicated porcine thyroid are also marketed for this indication.