Thyroxine Vs. Triiodothyronine in the Fowl

Following the discovery of a previously unrecognized iodinated compound in mammalian thyroid and plasma (Gross et al. 1950), Gross and Pitt-Rivers (1952) identified the compound as 3:5:3′-l-triiodothyronine. Many subsequent investigations have shown that l-triiodothyronine (T-3) is several times as potent as l-thyroxine (T-4), as judged by various criteria in mammals and by tadpole metamorphosis (Roche and Michel, 1955). In experiments in our laboratory involving radiochromatography, L. B. Hardy, Jr, identified both T-3 and T-4 as components of chicken plasma in 1957 (unpublished data); and in 1958, one of us (B. C. W.) found T-3 and T-4 in both plasma and thyroid extracts from chickens (unpublished data). Shellabarger and Pitt-Rivers (1958) have also isolated both T-3 and T-4 from chicken thyroids. That the relative potencies of T-3 and T-4 might not be the same in the fowl as in mammals was indicated by Shellabarger (1955), who reported that T-3 was no …