Levothyroxine sodium tablets: chemical equivalence and bioequivalence

Two brands of levothyroxine sodium tablets were compared in vivo for bioequivalence in a double-blind, randomized study. The tablets were identical in levothyroxine content. Evaluation was by means of triiodothyronine (uptake), tetraiodothyronine, free thyroxine index, total triiodothyronine by radioimmunoassay, and thyroid-stimulating hormone measurements. No differences in clinical response were found in a study with a high statistical power. It was concluded that the two brands were bioequivalent because of chemical equivalence, use of micronized levothyroxine powder in tablet production with at least one of the brands, and scrupulous attention to quality control during the manufacturing process, all of which contributed to assurance of homogeneity of the products and close adherence (+/- five percent) to the claimed potency.