After having reviewed the scientific literature and exchanged experiences between physicians from all over the world who are competent in hormone therapies, we, members of the Consensus Group of Experts of the International Hormone Society, think the time is ripe to reconsider current concepts on thyroid treatment of hypothyroidism.
The view that hypothyroidism is best treated by thyroxin alone is not based on solid scientific evidence. The studies comparing the efficacy of thyroxin alone versus that of combination thyroxin and triiodothyronine medications have in general not shown superiority of thyroxin alone above the combination of thyroxin with a smaller dose of triiodothyronine. On the contrary, a few studies have shown a significantly greater efficacy of combined thyroxin-triiodothyronine medications compared to the use of thyroxin alone in humans on such divergent parameters as serum cholesterol, mental and physical symptoms, and in animals on goitre formation and intracellular triiodothyronine (T3)-euthyroidism, just to name some of the greater benefits. The facts that T3 is the major intracellular thyroid hormone, and that a low serum level of T3 is more often than a low serum T4 (thyroxin) or a high TSH, the critical parameter in mortality studies, especially cardiovascular, and that the absorption of T3 is much more efficient and stable than that of T4, give credibility to the view that a combination of thyroxin with triiodothyronine may be better for the hypothyroid patient.
Read the entire document from the International Hormone Society's Consensus Group.