Innovative formulations for the delivery of levothyroxine to the skin.

The aim of this work was to realize innovative transdermal formulations containing sodium levothyroxine in view of topical administration. Permeation experiments were performed in vitro, using rabbit ear skin as barrier. At the end of the permeation experiments levothyroxine retained in the skin was extracted and quantified by HPLC. Formulations tested were microemulsions and transdermal films. Microemulsions containing isopropyl myristate and isobutanol were shown to be able to increase levothyroxine solubility by the inclusion in reverse micelles. However, the inclusion in reversed micelles reduced the drug release to a significant extent, and consequently skin retention, compared to aqueous solutions. When the microemulsion was included in the transdermal film, drug retention was increased, probably for the enhancer effect of its excipients. The transdermal film proposed in this work could be an interesting alternative to semisolid formulations for the ease of use and the control in the amount of active applied. Additional benefit can be obtained if the film is used in occlusive conditions.