Effects of maternal iodine deficiency on the L-thyroxine and 3,5,3'-triiodo-L-thyronine contents of rat embryonic tissues before and after onset of fetal thyroid function.

Female rats were placed on a low iodine diet (LID) or LID supplemented with KI. They were mated 3-6 months later. Maternal and embryonic tissues were obtained both before the onset of fetal thyroid function, at 11 and 17 days of pregnancy, and at 21 days of gestation. T4 and T3 concentrations were measured by RIA. T4 concentrations were very low in the plasma, liver, and lung of LID dams and in all embryonic samples obtained from such mothers, namely 11-day-old embryotrophoblasts, 17-day-old placentas and embryos, 21-day-old placentas, embryos, plasma, liver, lung, and carcass (whole embryos minus the trachea, thyroid, blood, liver, and brain). T3 was low in 17-day-old placentas and embryos and in all fetal tissues obtained at 21 days of gestation from LID dams. These results show that when iodine deficiency is severe enough to result in very low maternal plasma T4 levels, embryonic tissues are deficient in T4 and T3 both before and after the onset of fetal thyroid function. This finding might be relevant to the etiopathology of human iodine deficiency disorders.