Development of Regulatory Mechanisms in the Thyroid: Failure of Iodide to Suppress Iodide Transport Activity

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Abstract
Fetal, neonatal and adult cat thyroid glands were investigated for their ability to respond to excess iodide in vitro with reduction in both iodide transport activity as measured by the 125I-T/M [thyroid medium] ratio and TSH-stimulated cAMP formation. Fetal and neonatal cat thyroids responded to TSH-addition with greater increases in cAMP than those of adults. Moreover, in each group, pretreatment of the thyroid tissue with NaI (30 .mu.mol for 2 h) resulted in a significant reduction in the subsequent stimulation of cAMP formation by TSH. The 125I-T/M ratio in neonatal cat thyroid tissue was similar to that in adult animals of either sex, whereas the 125I-T/M of fetal thyroid was greater than that of the mother. Moreover, while pretreatment of the thyroid tissue of neonatal or adult animals with excess iodide resulted in a reduction in iodide transport activity, no such reduction was observed in fetal thyroid tissue. Apparently the autoregulatory effect of iodide transport develops at a later gestational time than the action on TSH-stimulated cAMP formation.