The effect of iodine deficiency on thyroid function in the infant rat

We have studied rats born to severely iodine-deficient mothers and subsequently maintained on a low-iodine diet (LID) from birth to 41 days. They were compared with controls born to mothers fed a high-iodine diet (HID). LID babies from birth on had large goiters, high thyroid labeled MIT/DIT ratios and radioiodine uptakes, high plasma TSH and low plasma T4 in comparison to HID controls. Thyroid labeled T3/T4 ratios were low in all babies at birth but were higher in the LID than HID babies from day 5 on and were greater than 1 after day 10, approximating the T3/T4 of the mothers. Coupling efficiency, as indicated by thyroid labeled (T3 + T4)/(MIT + DIT), was relatively low for the first 4 postnatal days for both the HID and LID babies and was associated with a lower plasma T4 than at later intervals. Injection of 0.05 mug 127I- simultaneously with 131I- caused an acute increase in labeled T4 and decrease in MIT formation in LID babies of all ages, but did not affect T3 synthesis. Doses of 127I- 20 times as large had no effect on labeled iodoamino acid synthesis in HID babies. Thyroid organic radioiodine content in newborn LID rats was 65% lower at 24 hr than at 4 hr after 131I injection, indicating that thyroid secretion was occurring. A few of the LID pups were "runts" approximately 60% the size of HID babies the same age. However, the overwhelming majority of LID babies maintained the same weight as HID controls from birth until weaning. After weaning, the LID babies grew at a slower rate than the HID controls. Relative thyroid weight, radioiodine metabolism and plasma TSH were no different in runts of various ages or in their mothers than in the "normal" LID controls. Adaptation was apparently adequate in the LID babies to maintain a nearly euthyroid state. We suggest that the low labeled T3/T4 ratio in the first few days of life in the LID babies may be due to a coupling deficiency in newborn rats resulting in a proportionately greater formation of iodotyrosines than of iodothyronines compared to older animals. This results in a greater intrathyroidal retention of iodine during intracellular thyroglobulin proteolysis and a more highly iodinated thyroglobulin in the LID babies than after normal coupling is achieved.