Is Long-term Levothyroxine Therapy Safe?

Our understanding of thyroid hormone action has increased tremendously since 1891, when George Murray1first successfully treated a hypothyroid patient with sheep thyroid extract injections. As time and experience progressed, we have learned that the thyroid gland synthesizes and secretes two major active components, thyroxine and triiodothyronine. In fact, each of these products has been synthesized and is used now for the treatment of various clinical conditions. Presently, thyroid preparations are prescribed frequently for a variety of common clinical conditions (eg, hypothyroidism, suppression of solitary nodules, or multinodular goiter) See also pp 2077-2081. and, in 1988, accounted for about 1.3% of prescriptions filled in the United States (S. C. Kaufman, MD, T. P. Gross, MD, and D. L. Kennedy, MPH, unpublished data, 1990).2Endocrinologists are virtually unanimous in recommending synthetic levothyroxine for long-term use, and about 70% of prescriptions written for thyroid preparations in the United States by