Kinetics of degradation of levothyroxine in aqueous solution and in solid state.

The kinetics of the deiodination of levothyroxine in aqueous solution were studied over the pH range 1 to 12. Temperature dependence of the reaction was also studied. The log k - pH profile indicated that the kinetics of deiodination include proton attack on the anion and dianion in acidic solution and water attack on the anion and dianion in alkaline solution. A possible mechanism of the deiodination was discussed. The solid-state stability of levothyroxine sodium was studied at elevated temperatures; and the compound was found to undergo deamination on heating. The decomposition follows biphasic first-order kinetics, with the most rapid decomposition occurring at the beginning of heating.