Identification of the Effects of Thyroxine Mediated by the Hypophysis

ACCORDING to the concepts of Classical Endocrinology, an effect of thyroxine may be defined as the restoration by this hormone1 of a physiological process altered by thyroidectomy. Such effects are not necessarily due to a direct action of thyroxine on an end organ; some may result from complex hormonal interactions. Thus, Evans and his associates have suggested that the growth-promoting effect of thyroxine is exerted through the hypophysis (Scow et al., 1949; Koneff et al., 1949; and Becks et al., 1950). It was decided to examine systematically the relationship between the hypophysis and the production of the effects of thyroxine. To this end, hypophysectomized animals were treated with replacement doses of thyroxine and the results obtained were compared with those observed in thyroidectomized animals similarly treated. The conclusions derived from this comparison helped to clarify the role played by the hypophysis in the production of thyroxine effects.