Context: In animals, acute iodine administration results in acute intrathyroidal inhibition of iodinations followed by escape of the inhibition, if the excessive iodine intake continues. In man the intrathyroidal non-hormonal and hormonal iodine concentration after exposure to large doses of iodine for a relatively long period of time is not known.

Objective: To determine whether in human thyroid, administration of large doses of iodine for a relatively long time results in alterations of intrathyroidal hormonal (HI), T₄ and T₃ and Total Iodine (TI) content, as well as weather changes in serum concentration of thyroid hormones and TSH would occur after iodine administration or discontinuation

Design: In thirty-three euthyroid patients suffering from single thyroid nodule or hyperparathyroidism, Lugol solution (80 mg iodine) was administered for fifteen days before operation. Groups of six to eight patients were operated the day 0, 5, 10 and 15 after iodine withdrawal. TI, HI in a sample of thyroid tissue, and serum concentration of T₄, T₃ and TSH were measured. In twenty-one normal euthyroid subjects who were not operated, a similar protocol was employed and serial blood measurements were done.

Main outcome measure: Intrathyroidal TI, HI, and serum thyroid hormone and TSH measurements were the main outcome measure.

Results: Intrathyroidal HI content and serum T₄ and T₃ were unchanged during and after iodine discontinuation. TI was increased during iodine administration and returned to control values 5 days after discontinuation of iodine. The ratio of HI/TI was decreased and returned to control values 15 days after the iodine was discontinued. Serum TSH was increased during iodine administration and returned to control values10 days after iodine withdrawal.

Conclusions: In man, administration of iodine for a relatively long period of time was accompanied by increased intrathyroidal TI but no changes in HI nor demonstrable increases of serum T₄ and T₃ were observed. It is hypothesized that the maintenance of normal intrathyroidal HI is the result of the combined inhibitory effect of iodine on thyroid hormone synthesis and on the release of T₄ and T₃ from the thyroid.