Thermal stability of synthetic thyroid hormone L-thyroxine and L-thyroxine sodium salt hydrate both pure and in pharmaceutical formulations

In this paper, the thermal stability of pure L-thyroxine (THY) and L-thyroxine sodium salt hydrate (THYSS) vs. two pharmaceutical solid formulations commercialized on both Romanian and European market (with a content of 100 mu g, respectively 200 mu g THYSS per tablet) were investigated. In order to determine whether the presence of excipients affects the thermal stability of the active pharmaceutical ingredient (API), the preliminary study of thermal stability in air atmosphere was completed with an in-depth solid-state kinetic study. By kinetic analysis, the non-isothermal degradation of the selected active pharmaceutical ingredients vs. the solid formulation with strength of 200 mu g THYSS per tablet was investigated. Isoconversional methods (Kissinger-Akahira-Sunose, Flynn-Wall-Ozawa and Friedman) were employed for the estimation of activation energies values, at five different heating rates, beta =5, 7, 10, 12 and 15 degrees C min(-1). Also, a fourth method was applied in the processing of data, namely NPK, allowing an objective separation in the physical and chemical processes that contribute to the thermal degradation of the selected compounds. A discussion of thermal stability from the kinetic point of view is also presented. (C) 2016 Elsevier B.V. All rights reserved.