[Diurnal rhythms of thyroid hormones in rooster chicks (Gallus domesticus). 2. Studies on the protein-bound radioiodine and the "free" hormone portion].

In four week old cockerels the plasma was investigated for cyclic changes in total radioactivity, PB131I, PI131I, and "free" thyroid hormones 24 hours after injection of Na131I. Maxima in total radioactivity were observed at 3.00 am and 9.00 pm. They were significant different from the minima at noon (2P less than 0.005). At the same points of the day maxima, resp. minima were found in the PB131I and the "free" hormones. The "free" hormones expressed in per cent of total hormones showed low values at 6.00 am, 3.00 pm, and 9.00 pm. The diurnal changes in haematocrit and albumin concentration were not responsible for the variation of PB131I. In order to eleminate effects of isotope dilution by ingested iodine the PB131I was expressed in per cent of total radioactivity of plasma (= conversion rate) or resp. in per cent of thyroidal radioiodine uptake. The obtained values showed maxima at 6.00 am and 9.00 pm. From that we conclude again on a stimulation of the thyroid at these times.