The crystal and molecular structures of 3,5,3′–triiodothyropropionic acid (T3P), determined as an N-diethanolamine salt, were carried out and the results are compared with those of other thyroid hormone structures. These data show that T3P has an unusual conformation with the diphenyl ether bridge outside the range normally observed for other thyroactive acid structures and has the largest deviations from the ideal skewed conformation predicted for 3,5–diiodothyroactive compounds. These conformational properties are not observed in the structures of thyroformic or acetic acid analogues. Biochemical data indicate that thyropropionic acid analogue activity differs from that of other acid analogues which could imply that their metabolism and activity can be controlled differently from that of other hormone metabolites.