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Three-dimensional structure of L-thyroxin.

The biological importance of L-thyroxin, the principal hormone secreted by the thyroid gland, is well established, but the mechanisms of thyroid-hormone action are still largely unexplained. We have crystallized L-thyroxin and determined its three-dimensional structure by x-ray crystallography in the hope that comparison of structures of active and inactive thyroid-hormone analogues will shed some light on hormone mechanisms. We compare the conformation of L-thyroxin to that of triiodo-L-thyronine, a thyroid hormone with greater activity.