EFFECTS OF SELENIUM AND IODINE DEFICIENCY ON THYROID-HORMONE CONCENTRATIONS IN THE CENTRAL-NERVOUS-SYSTEM OF THE RAT

Objective: The effects of single and combined nutritional selenium and iodine deficiency on intracellular thyroid hormone concentrations and type II 5'-iodothyronine deiodinase (S'D-II) activity were examined i n different regions of the adult rat brain. Design: Four groups (n=6) of weanling female Wistar rats proceeding from a breeding line fed a s elenium-deficient or a selenium-replete diet for 3 generations, were f ed selenium-deficient, iodine-deficient, combined selenium- and iodine -deficient or selenium- and iodine-replete diets for 2 months before t hey were killed. Methods: Tissue thyroxine (T-4) and tri-iodothyronine (Ts) concentrations were determined by highly sensitive RIAs after ex traction of the iodothyronines from the tissue samples. The measuremen t of 5'D-II was based on the release of radioiodide from the I-125-lab elled substrate. Results: Selenium deficiency significantly decreased tissue Tg concentrations in the hippocampus, hypothalamus and striatum to 70-80% of controls, whereas no significant changes were found in t he cerebellum, cerebral cortex and brain stem. Tissue T-4 concentratio ns were only marginally affected with the exception of a 35% increase in the cerebral cortex. Iodine deficiency dramatically diminished seru m T-4 levels as well as intracellular T-4 concentrations in all region s examined up to 10-30% of control. In spite of a threefold enhancemen t; of 5'D-II, the iodine-deficient animals still had a significant red uction of tissue T-3 concentrations (50-65% of controls) in all region s excepting the cerebellum. The combination of selenium and iodine def iciency did not significantly alter this pattern of changes. Conclusio ns: These findings suggest that prolonged selenium deficiency as well as iodine deficiency may compromise thyroid hormone homeostasis in the adult brain leading to tissue hypothyroidism and therefore to impaire d brain function.