Serum Pattern of Thyroxine (T4), 3,3‘,5‐Triiodothyronine (T3) and 3,3’,5‘‐Triiodothyronine (rT3) in Fed and Fasted Cocks Following TRH Stimulation

Abstract

Food deprivation for 27 and 75h in cocks decreased serum levels of T3 maximally by 61.1% and increased the level of rT3 by 51.4% and that of T4 by 22.4%. Injection of a single dose of TRH (10 micrograms/kg b.w.) increased serum levels of all 3 iodothyronines in both fed and food-deprived animals. In fasted, TRH-treated birds the peak of serum rT3 was 80.5 and 73.3% above control levels in 27- and 75-hr food-deprived cocks, respectively (fed birds: 14.6 and 9.7%); the relevant data for T3 were: 78.3 and 40.4% (fed birds: 95.7 and 127.5%); for T4: 30.0 and 27.0% (fed birds: 9.2 and 6.1%). The greater relative increment of serum rT3 than T3 in food-deprived and TRH-treated cocks was supported by a fall of the serum T3/rT3 ratio to 78.7 and 27.9% in fed cocks and 27- and 75-hr fasted animals, respectively. We conclude, that food deprivation modifies the response of the hypophysis-thyroid axis to exogenous TRH in the sense that during fasting the production of rT3 is enhanced relative to T3.