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The Role of Serum Binding Proteins in Determining Free Thyroid Hormone Concentrations during Development in Quail^*

F. M. ANNE McNABB and THOMAS E. HUGHES

Department of Biology, Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061

Abstract

Japanese quail were used as a model for studying the role of binding proteins in determining free T₄ (FT₄) and free T₃ (FT₃) concentrations during development. Adults were used to characterize thyroid hormone binding; developmental stages studied were late embryonic, perinatal, hatchling, and juvenile. Total and free hormones were determined directly by RIA, and free hormones were determined indirectly by equilibrium dialysis. Binding proteins were identified by electrophoresis of serum preincubated with labeled hormones.

Serum FT₄ and FT₃ concentrations in adult quail were equivalent to those in humans. T₄ bound principally to albumin and secondarily to prealbumin; T₃ bound principally to a-globulin and secondarily to albumin and -globulin. A specific T₄-binding globulin, as in mammals, was not present. The relative affinity of stripped serum was greater for T₄ than for T₃.

In late embryos, FT₄ concentrations rise as a result of a marked increase in total T₄ (TT₄) and modest increases in binding proteins. The perinatal peak in FT₄ reflects the perinatal surge of TT₄ without a change in binding proteins. From days 1–6 posthatching, FT₄ decreases as a consequence of TT₄ decreasing faster than the decrease in binding. In juveniles, FT₄ concentrations stabilize as increases in TT₄ are paralleled by increases in serum binding. T₃ binding shows few significant differences from adult values during development, so FT₃ concentrations follow closely the pattern of TT₃ changes. These results demonstrate that developmental changes in serum binding proteins play a significant role in determining the pattern of free thyroid hormones, especially for FT₄, by modulating the total hormone concentrations controlled by the hypothalamic-pituitary axis. (Endocrinology 113: 957, 1983)

Footnotes

^* This work was supported by Grant R01-AM-28216 from the NIH and a Biomedical Research Support Grant administered by VPI and SU from the NIH.

To whom requests for reprints should be addressed.

Received March 28, 1982.