Thyroid hormone preparations are among the most commonly prescribed medications in this country: in 1986, an estimated 20 million prescriptions were written for various forms of thyroid supplementation. Thyroid hormone formulations and the indications for thyroid hormone therapy have changed dramatically over the years, reflecting refinements in the understanding of thyroid physiology and thyroid hormone pharmacology. These changes have contributed to the bewildering array of thyroid hormone preparations that are available to the practitioner, as well as uncertainty about the proper therapeutic use of thyroid hormone.
Thyroid hormone preparations can be classified into two broad categories: synthetic and biologic. Synthetic thyroxine (T4) is the preferred form of thyroid hormone for almost all therapeutic indications. Because of its 1-week half-life, thyroxine yields relatively stable blood levels, with only minimal daily fluctuations in serum T4 concentrations. Furthermore, since peripheral tissues generate triiodothyronine (T3) from circulating T4, a preparation