The adaptation of the human thyroid gland to a physiological regimen of iodide intake: evidence for a transitory inhibition of thyroid hormone secretion modulated by the intrathyroidal iodine stores.

The effects of iodide on thyroid function were so far only studied after administration of pharmacological or small continuous doses. We were interested to know how the human thyroid gland would react to a more physiological situation: small doses taken in intermittently. Ten normal male volunteers were given weekly doses of iodide: 1 mg for the first 6 weeks and 2 mg afterwards for another 6 weeks period. Intrathyroidal iodine stores were evaluated by the X-Ray-Fluorescence method. The following thyroid parameters were estimated during the 3 months period: total thyroxine, total triiodothyronine, free thyroxine, free triiodothyronine, reverse triiodothyronine, plasma inorganic iodide, thyroid stimulating hormone and thyrotropin-releasing hormone tests. The following observations were made: i) a steady increase of intrathyroidal iodine (p less than 0.01); ii) no changes in free thyroxine, free triiodothyronine, thyroid stimulating hormone, thyrotropin-releasing hormone test or plasma inorganic iodide. iii) a decrease of total thyroxine (and total triiodothyronine) with a nadir at about 3 weeks and a spontaneous rise afterwards; repetition of this phenomenon by doubling the dose of iodide. It is concluded that 1 mg or 2 mg iodide a week does not inhibit incorporation into the normal human thyroid gland and suggested that these physiological doses of iodide cause a transitory inhibition of thyroxine secretion, representing a form of autoregulation of the thyroid cell, since it was modulated by the intrathyroidal iodine stores and no evidence of pituitary mediation could be evidenced in the experimental protocol.