In vitro permeation of levothyroxine across the skin

The aim of this work was to investigate the in vitro transdermal permeation characteristics of sodium levothyroxine, in view of its topical application. Permeation experiments were performed in vitro, using rabbit ear skin as barrier. At the end of the experiments levothyroxine retained in the skin was extracted and quantified by HPLC. The formulations tested were solutions and a commercial cream. The use of dimethyl β-cyclodextrin as solubilizing agent increased to a significant extent levothyroxine solubility, but reduced its skin accumulation. Skin stripping before drug application produced a considerable increase in the amount retained and levothyroxine was found also in the receptor compartment. The application of the commercial cream in occlusive conditions increased to a significant extent drug retention in the skin. In conclusion, levothyroxine skin administration is promising in view of a localized effect, because it was retained in the skin. On the contrary, transdermal administration in view of systemic effect does not represent a concrete possibility.