Thyroid Hormone Replacement — One Hormone or Two?

Extracts of animal thyroid tissue, first used in 1892, contained both thyroxine and triiodothyronine and were the only available treatment for hypothyroidism for some 50 years. Because of concern about their variable potency,1 these extracts have been considered obsolete for some time by all but a few practitioners, who are often thought by their colleagues to be practicing on the fringes of medicine. Treatment with thyroid extract (or synthetic triiodothyronine) also led to high serum concentrations of triiodothyronine, because of its rapid absorption, and some patients had palpitations one to two hours after taking it. Synthetic thyroxine did not have . . .