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April 16, 1997

Bioequivalence of Levothyroxine Preparations for Treatment of Hypothyroidism-Reply

Author Affiliations

University of California, San Francisco
Thomas Jefferson University Philadelphia, Pa

JAMA. 1997;277(15):1199-1200. doi:10.1001/jama.1997.03540390029020

In Reply.  —The letter from Dr Spigelman contains several inaccuracies and misconceptions.Treatment guidelines from the leading thyroid societies all were prepared before publication of current data so that current guidelines may need to be updated.1,2 It should be noted that these treatment guidelines do not actually recommend that retitration is necessary once levothyroxine brands are changed. Previous reports of levothyroxine nonequivalence studied products that were formulated before the adoption of the US Pharmacopeia (USP) guidelines. These guidelines require all manufacturers to monitor and assure the tablet levothyroxine content, thus reducing the potential for tablet subpotency to affect thyroxine equivalence. The high-performance liquid chromatography analysis of the tablet levothyroxine content of our study preparations, despite some variability in our assay, met FDA specifications for proper levothyroxine content. Slight differences in tablet content did not significantly change our results or the analysis by Mayor et al.3 Clinical interchangeability in patients given