[Differences in the effectiveness of levothyroxine preparations].

Three proprietary preparations of L-thyroxine were tested for their bioequivalence in a cross-over trial. Comparing the two most commonly prescribed preparations, during administration of preparation B (Euthyrox) serum concentrations of basal thyrotropin (TSH) was significantly higher than with preparation A (L-Thyroxin Henning) in both subgroups and in the cross-over calculation, as well as after stimulation with thyrotropin-releasing hormone (TRH) in one of the subgroups. During administration of a third preparation, C (Levothyroxine, enos), serum concentrations of total thyroxine (T4) and free thyroxine (fT4) were significantly lower than with preparation A, while basal and TRH-stimulated TSH levels were in all measurements highly significantly higher. Although there was the tendency for preparation B to be inferior, a greater effectiveness of preparation A could not be statistically proven for all data. But a markedly reduced bio-availability or effectiveness was demonstrated with respect to preparation C.