Despite the presence of considerable data supporting the superiority of synthetic levothyroxine sodium over animal thyroid gland extracts as thyroid hormone therapy, numerous physicians continue to prescribe desiccated thyroid preparations for their patients with hypothyroidism, colloid goiter, or both.
Historically, the initial criticism of desiccated thyroid as a therapeutic preparation had to do with the lack of purity, standardization, and stability. The United States Pharmacopoeia requires that desiccated thyroid possess a total organically bound iodine content of 0.2% ± 0.03%, without specifying the breakdown among the various compounds that may constitute this total.1 Clearly this requirement does not standardize biologic activity. For one thing, the ratio of thyroxine (T4) to triiodothyronine (T3) varies from one animal source to another,2 and these hormones are not equivalent, with respect to either biologic activity or iodine content. Thus, uniform biologic potency among various preparations or lots of desiccated thyroid