Respective roles of circulating T4 and T3 in control of TSH secretion in severely iodide-deficient rats

Summary After a 6-month iodide deficiency, Wistar male rats were submitted to a normal iodine diet (20 and 50 g of¹²⁷I daily). Plasma T₃, T₄ and TSH were determined by RIA from 0 to 140 days of iodide refeeding. A highly significant correlation was found between plasma TSH and T₄ concentrations, but not between plasma TSH and T₃ levels. These data suggest that an increase in plasma T₃ alone, up to the normal value, is not able to inhibit TSH secretion. It is only when a certain plasma T₄ concentration is also reached, resulting in further T₃ formation through deiodination, that TSH secretion is inhibited.Dedicated to the memory of Prof. C. Simon.This work was supported by grants from the CNRS (Equipe de Recherche Associée no 234) and from the INSERM (A.T.P. 49.77.8).Acknowledgments. The authors wish to thank Mrs M. Chartier for valuable technical assistance. They are indebted to the rat pituitary distribution programme of NIAMDD, NIH, Bethesda, for their gift of rat TSH reagents.