Excess iodide and thyroid function in the developing chick embryo.

The effects of adding excess iodide (I-) to egg yolks were studied on the quantity of iodide transferred to the chick embryo and on thyroid function from Days 8 to 18 of incubation. Following injection of either 0.5 or 1 mg of iodide into the yolk on the second day of incubation, embryonic plasma iodine levels increased significantly over levels of controls at the same age. These increases were proportional to the amount of excess iodide in the yolk. Moreover, a constant ratio between the iodine levels of the allantoic fluid and the plasma was maintained for any given day of incubation. Up to Day 10, the iodine content of the embryonic thyroid was closely correlated with the increased plasma iodine concentrations. After Day 10, concomitant with the establishment of the hypothalamo-adenohypophyseal-thyroid axis, the increase in thyroid iodine no longer paralleled that of plasma iodine, but was only twice that of controls. As thyroid weight doubled in these treated embryos, thyroid iodine concentrations of control and treated embryos were similar. Iodide excess did not block thyroid iodine organification, nor was there any modification of iodine distribution among the different iodo-amino acids. Moreover, despite the doubling of thyroid hormone content in the goiters of treated embryos, circulating levels of thyroid hormones in control and treated embryos were not different.