he exchange of radioiodine between maternal blood, fetal blood, and amniotic fluid was studied in sheep in the 120-125th day of gestation. A double-tracer technique, using 1311 and 1251, and a surgical procedure for in utero sampling of fetal blood and amniotic fluid permitted quantitation of the radionuclides transferred between the fluid compartments up to 48 hr after injection. Rapid exchange of the radioiodines was observed between damand fetus and between fetus and amniotic fluid. Regardless of the injected compartment, the 48-hr concentration of radioiodine in amniotic fluid exceeded that of fetal blood which, in turn, was greater than that in maternal blood; the concentration in thyroid glands varied with the injection site. Maternal and fetal thyroid glands contained 14 and 7.7% of the injected dose, respectively, after maternal injections; 5.7 and 4.2% after fetal injection; and 0.9 and 1.5% after amniotic fluid injection. Fetal/maternal ratios of radioiodine per gram of thyroid gland were 4.8 after maternal injection, 6.2 after fetal injection, and 21 after injection into the amniotic fluid. The amniotic fluid may serve as a reservoir for iodine for the fetus, accounting for the greater concentration of radioiodine in the fetal thyroid gland compared to the dam's in advanced pregnancy.

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