Since the isolation of triiodothyronine (T3) by Pitt-Rivers and Gross (1953) several questions as to its role as a thyroid hormone arose. Does T3 have the same or different actions than that of thyroxine (T4 )? Is T3 an emergency hormone, since its synthesis relative to T4 is greatly increased during periods of low iodine intake and its response time is I/5th that of T4 (Ackerman and Arons, 1958)? Are there any interactions modifying their respective actions?