Levothyroxine: therapeutic use and regulatory issues related to bioequivalence.

Levothyroxine is the overwhelming choice of clinicians for the treatment of hypothyroidism and for the suppression of goitre and thyroid nodules in selected cases. The monitoring of serum levels of thyroid stimulating hormone is necessary for appropriate dosage adjustment of levothyroxine. Levothyroxine has a narrow therapeutic index: both underdosage (subclinical hypothyroidism) and excessive dosage (subclinical hyperthyroidism) are associated with adverse symptoms and pathophysiological effects and are to be avoided. The consequent necessity for careful titration of doses has had an impact on the issue of switchability, or bioequivalence, of the various marketed levothyroxine products. In this article, the basis for concern about currently accepted standards of the FDA for pharmacological bioequivalence are examined in the context of levothyroxine. The history and status of the recent request by the FDA for a new drug application for all levothyroxine products, and its impact on the market leader Synthroid, is also discussed.