New research has found that some patients who continue to experience symptoms of hypothyroidism despite receiving conventional therapy may benefit from administration of a synthetic form of the thyroid hormone triiodothyronine (T3). Hypothyroidism is a disorder that occurs when the thyroid gland does not make enough T3 to meet the body’s needs. T3 regulates metabolism—the way the body uses energy—and affects nearly every organ in the body. Without enough of the hormone, many of the body’s functions slow down. The disorder—which can stem from a variety of causes—affects about 4.6 percent of the U.S. population age 12 and older. Symptoms of hypothyroidism include fatigue, weight gain, facial “puffiness,” intolerance of cold, and many other problems. The thyroid secretes T3 along with thyroxine, also known as T4, a less potent form of thyroid hormone, which is converted to T3 by various tissues throughout the body. Because T4 is more stable than T3, and the body can generate T3 from T4, current standard therapy for hypothyroidism is a once-daily pill containing a synthetic form of T4.
However, experiments have shown that not all tissues in hypothyroid mice dosed with T4 achieve as high a level of T3 as they need, which may explain why a subset of people with the disorder continue to experience some of its symptoms even when receiving what should be an adequate dose of T4. Previous studies have tested T3/T4 combination therapy, but the results of these studies have been inconsistent. The present study compared therapy with T4 alone to therapy with T3 alone in 14 participants who had continued to experience some symptoms of hypothyroidism while receiving standard therapy. The study was designed so that after 6 weeks those receiving T3 were switched to T4, and vice versa—with neither the patient volunteers nor the researchers knowing until after the trial was over which participants received T3 in the first half of the trial, and which received it second. Participants took the medicines orally, three times per day, before meals. The researchers found that participants lost weight on T3 relative to T4, showed significant improvement in their levels of cholesterol and other blood fats, and experienced no serious side effects. A longer, larger trial will be necessary to determine the long-term safety and efficacy of this approach. However, this preliminary study suggests that while T4 remains an excellent approach for most people with hypothyroidism, particularly given the relative convenience of its
once-daily dosing, thrice-daily dosing of T3 might be a good alternative for those who continue to experience symptoms.
Celi FS, Zemskova M, Linderman JD, et al. Metabolic effects of liothyronine therapy in hypothyroidism: a randomized,
double-blind, crossover trial of liothyronine versus levothyroxine. J Clin Endocrinol Metab 96: 466-3474, 2011.