Iodine supply to the thyroid gland in the developing chick embryo.

During embryonic development of the chick it was found that 81% of the iodine deposited in the egg yolk, principally in the form of iodine, I-, was transferred to the embryo. Changes in plasma iodide levels, [I-], showed that transport is low between days 8 and 9 of incubation ([I-] = 2 micrograms/dl) and increased from day 10 ([I-] = 3 micrograms/dl) to day 20 ([I-] = 77 micrograms/dl). This increase of [I-], coupled with an increase in total blood flow through the thyroid gland, results in an increasing supply of iodide to the thyroid throughout the incubation period. Our results and those of other workers suggest the existence of bidirectional exchanges of iodide between the embryo and the yolk which may be under pituitary control.