Thermal inactivation of L-thyroxin.

We assessed the extent of inactivation of L-thyroxin induced by exposure to heat in the presence of two vehicles. Preparations of L-thyroxin in the dry powder form, or dispersed in the solvents propylene glycol (water-like) or ethoxylated castor oil (oil-like), were heated at temperatures ranging from 65 to 160 degrees C, for 5- to 15-min periods. Heating L-thyroxin to a temperature below that of cooked bovine ground meat (72 degrees C) produced less than 10% degradation. Thermal degradation was pronounced only above 90 degrees C, and was almost completed at 160 degrees C. L-Triiodothyronine was the only thermal degradation product identified after L-thyroxin was heated at 125 degrees C. In a separate experiment we measured the melting point of L-thyroxin, 148.81 degrees C. This value agrees closely with the observed thermal sensitivity. We conclude that L-thyroxin is not significantly degraded under conditions encountered during cooking of ground bovine meat for short times at moderate temperatures.