A HIGH IODINE INTAKE IN WISTAR RATS RESULTS IN THE DEVELOPMENT OF A THYROID-ASSOCIATED ECTOPIC THYMIC TISSUE AND IS ACCOMPANIED BY A LOW THYROID AUTOIMMUNE REACTIVITY

Evidence is accumulating that dietary iodine intake is an important mo dulator of autoimmune thyroid reactions. To study this role of iodine intake further, female Wistar rats were kept on an enriched iodine die t (EID, iodine intake 100 mu g iodine/day) for a period of up to 18 we eks. Control rats were either on a normal iodine diet (NID, iodine int ake 7 mu g iodine/day) or a low iodine diet (LID, 2 days of 1% KC10(4) followed by iodine-deficient drinking water/pellets). During the firs t 6 weeks of the EID rats developed a thyroid-associated ectopic thymi c tissue (50-57% of the animals on EID versus 7-14% of NID rats and 0% of LID rats). This thyroid-associated ectopic thymic tissue showed a similar histology (cortex and medulla) and a similar marker pattern as normal rat thymus concerning TdT expression (positive cells in the co rtex) and CD4/CD8 positivity (double-positive cells in the cortex, sin gle-positive cells in the medulla). The excessive iodine diet also res ulted in a lowered thyroid autoimmune reactivity as compared to the NI D and LID, viz. (1) in a lower incidence of anti-colloid antibodies in serum (125% positivity in EID rats versus 36% in NID and 60% in LID r ats at 18 weeks) and (2) lower numbers of intrathyroidal lymphoid cell s, viz. lower numbers of dendritic cells and lower numbers of CD4 and CD8 positive lymphocytes. It is hypothesized that the development of t he thyroid-associated ectopic thymic tissue in the EID rats is related to their low thyroid autoimmune responsiveness; the tissue might play a role in tolerance induction to thyroidal autoantigens.