To the Editor.
—Numerous published articles address the issue of levothyroxine bioequivalence. The conflicting results from these studies leave the closely related issues of bioequivalence and therapeutic interchange unresolved. This view is supported by treatment guidelines from leading thyroid societies as evidenced by their recommendation that patients switched from 1 levothyroxine preparation to another should be reevaluated, and if necessary retitrated.1,2 Thus, the published guidelines of these societies recognize that the issue of levothyroxine bioequivalence has not been adequately addressed, and the question remains unanswered whether any 2 preparations provide similar levels of serum thyroxine (T4) and, accordingly, normalize serum thyrotropin (TSH) to the same degree.See also pp 1205 and 1238.In this context, Dr Dong and colleagues3 assert their study shows that 4 levothyroxine preparations are bioequivalent. They should therefore be similarly effective and interchangeable in treating hypothyroidism, ie, normalizing TSH. However, the data of
Spigelman MK. Bioequivalence of Levothyroxine Preparations for Treatment of Hypothyroidism. JAMA. 1997;277(15):1199. doi:10.1001/jama.1997.03540390029019