Thyroid iodine uptake, thyroid iodine secretion and plasma TSH levels in male rats during the day and night.

Experiments were performed to assess whether the diurnal changes in thyroid 131I metabolism are paralleled by variations in endogenous iodine. The uptake of iodide by the thyroid gland was determined by a double isotope technique. The T/S iodine ratio and the rate of 131I and 127I secretion from the thyroid gland were determined in rats fed LID during 20 days. The accumulation of iodine by the thyroid gland was prevented by CK1O4 and organic iodine formation was blocked by PTU. The absolute uptake of iodide by the thyroid gland was twice as high during the night than during the day. The thyroid/serum iodide ratios were highest during the night. The rate of release of 131I and 127I from the thyroid gland was augmented in the afternoon. The concentration of TSH in the plasma, measured by radioimmunoassay, was elevated in the afternoon as compared to other times during the day and night. The close association between the diurnal changes in thyroid iodine secretion and plasma TSH concentrations suggest that TSH may be responsible for the differences in iodine secretion observed during the day and night. On the other hand, the intake of iodine in the diet and the urinary clearance of iodine showed diurnal rhythms; the highest values occurred during the night. In addition, about twice as much iodine was available to the thyroid gland at night than during the day. These factors might also contribute to the diurnal variations observed in thyroid iodine metabolism.