Comparison of Pituitary-Thyroid Maturation in the Fetuses of Rats Fed an Iodine-Deficient or Normal Diet*

From the earliest detectable development of fetal pituitary-thyroid function (day 18-19 of gestation) through the first postnatal day, there was a higher degree of stimulation of the pituitary-thyroid axis in the fetuses of rats fed a low iodine diet (LID) than in those of rats fed a high iodine diet (HID). Significant differences between the two groups were consistently observed in relative thyroid size, plasma TSH, 4-h thyroid radioiodine uptake, and the labeled iodoamino acid composition of thyroid digests. Plasma T4 concentration was lower in both LID and HID fetuses and pups than in the HID mothers. Plasma T3 was not detectable (less than 20 ng/dl) in the fetuses of either group, nor was labeled T3 in the thyroid digests. Body weight, plasma T4, and pituitary TSH content were usually lower in the LID than the HID animals of comparable age; however, these differences were not consistently statistically significant (P less than 0.05). We conclude that iodine deficiency causes a marked stimulation of TSH secretion and, consequently, of thyroid growth and metabolism from the earliest development of fetal pituitary-thyroid function.