Abstract
Production rates (PR, μg/M2/d) and metabolic clearance rates (MCR, L/M2/d) for thyroxine (T4), triiodothyronine (T3), and reverse T3 (rT3) were measured in 4 newborn sheep 7 to 14 days of age using single injection non-compartmental methods. Results were related to data in fetal and adult sheep. Thyroid secretion of T3 and rT3 were calculated from T3/T4 and rT3/T4 ratios measured in adult and fetal thyroid glands:
Newborn T4 MCR>fetal (p<.05) and adult (p<.01); newborn T4 PR> fetal (p<.05) and adult (p<.01). Fetal T3 MCR>newborn (p<.01) = adult; newborn T3 PR>fetal (p<.01)>adult (p<.001). Fetal rT3 MCR = newborn<adult (p<.05); fetal rT3 PR>newborn (p<.01) = adult. The T3 secreted from the thyroid is minimal in fetus, approximates 54% in newborn, and 51% in adult animals. Percent rT3 secretion approximates 3% in fetus, 18% in newborn, and 3% in adult animals. Conclusions: In the newborn a) T4 secretion is increased; b) T3 production is increased as a result of increased secretion and augmented T4-T3 conversion; c) rT3 production from T4 is decreased, and rT3 secretion is minimal; d) thyroid sensitivity to TSH or pituitary-thyroid feedback appears to be altered during this period.
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Klein, A., Padgett, D., Castagna, P. et al. 306 ONTOGENESIS OF IODOTHYRONINE PRODUCTION AND CLEARANCE IN SHEEP. Pediatr Res 12 (Suppl 4), 414 (1978). https://doi.org/10.1203/00006450-197804001-00311
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DOI: https://doi.org/10.1203/00006450-197804001-00311