Improvements in the determination of the turnover rate of plasma thyroxine in avian species: application to the Japanese quail.

The disappearance of the thyroid hormone thyroxine (T4) from plasma in fully grown male Japanese quail can be described as a first order process with a rate constant of 0.178 +/- 0.013/h (mean +/- S.E.M., n = 8), which represents a half-life of 3.90 h. A small amount of [125I]T4 in relation to total circulating T4 was injected i.v. into Japanese quail and plasma samples were taken at appropriate time-intervals for the determination of residual plasma radioactivity. The rate of disappearance of [125I]T4 was subsequently equated to the turnover rate of the endogenous hormone. Previous methods were modified to overcome problems arising from possible disturbance of plasma T4 metabolism, recirculation of radiolabelled iodide, and to purify the [125I]T4 from the plasma samples. By using labelled T4 of very high specific activity, the amount of [125I]T4 administered was kept much smaller than has been used in previous studies on Japanese quail, thus limiting any interference with plasma T4 dynamics. To minimize any disturbance of plasma T4 metabolism, only four blood samples were taken, at three-hourly intervals after the injection of [125I]T4. The rapid turnover of T4 produced a large amount of labelled inorganic iodide, the re-entry of which into the plasma T4 pool was inhibited by s.c. administration of sodium thiocyanate 1 h before injection of [125I]T4. Assay of the true [125I]T4 turnover was significantly improved over that used in previous studies by purifying the [125I]T4 from the plasma samples chromatographically. The samples were applied to small Sephadex G-25 columns with sodium hydroxide (0.1 mol/l) as the eluant.(ABSTRACT TRUNCATED AT 250 WORDS)