[Kinetics of thyroxine in ontogenesis studies using the metabolism of radiothyroxine].

Radiothyroxine turnover rate was studied postnatally in 9, 18, and 35 day old rats in various intervals during 24 hours after intrajugular injection. Value of fractional turnover rate indicated higher thyroid activity in 18 day old rats than in those aged 9 or 35 days. These results support our previous conclusions on the maturation of thyroidal regulatory mechanisms in this period of development which were based on the rate of radioiodine uptake and changes in serum thyroxine concentrations. In another series of animals, the distribution of radiothyroxine in comparison with distribution of radioiodide was studied 4 hours after intrajugular injection on days 8, 12, 16, 20, 24, and 30 of postnatal development. In case of hormone distribution, all values in the brain, liver, kidney, intestine, and serum were much higher in young animals than in adult 60 day old rats. Particularly, in the brain up to 16th day, the concentration of T4 was six times higher than in adult animals. The highest percentage of the injected radiothyroxine dose was found in the liver, intestine and stomach, total values in these organs exhibited a progressive increase from 20% on day 8 to 50% on day 24. When compared with litter-mates injected with inorganic radioiodide, the radioactivity in animals treated with hormone was in most organs markedly higher. The opposite results were found only in the stomach, i.e. the uptake of iodide exceeded the uptake of hormone. These results suggests, that differences between age groups are not due by differences in vascularity and although indirectly they may be considered as reflecting changes in size of thyroxine binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)