The changes of thyroglobulin in the lysosomal particles of thyroid during the postnatal period in rats.

The distribution of radioactivity and protein-SH groups was studied in neonatal rats injected with 125-I- in the 15,000 g pellets of thyroid glands to obtain information about the colloid which undergoes phagocytosis by lysosomes. The 15,000 g pellet was solubilized in Triton X--100. The soluble portion was enhanced on the 16th day of age, whereas the portion of the insoluble fraction remained almost unchanged. The content of protein-SH groups in the insoluble fraction decreased with age and was increased in the soluble fraction. The concentration of immunoreactive thyroid stimulating hormone (TSH) in the pituitary and in the blood of rats during ontogenesis was higher on the first day after delivery, then decreased within the first seven days and again markedly increased during the third postnatal week. It was concluded that higher amounts of Triton soluble thyroglobulin occurring during the postnatal period in rats reflects the availability of more substrate, i.e. thyroglobulin for pinocytosis and partially depends upon the TSH level. These findings also support the suggestion on maturation of the pituitary-thyroid system during the postnatal period in rats.