LOW SELENIUM STATUS IN THE ELDERLY INFLUENCES THYROID-HORMONES

1. Iodothyronine 5'-deiodinase, which is mainly responsible for periph eral triiodothyronine (T-3) production, has recently been demonstrated to be a selenium-containing enzyme. In the elderly, reduced periphera l conversion of thyroxine (T-4) to T-3 and overt hypothyroidism are fr equently observed. 2. We measured serum selenium and erythrocyte gluta thione peroxidase (as indices of selenium status), thyroid hormones an d thyroid-stimulating hormone in 109 healthy euthyroid subjects (52 wo men, 57 men). carefully selected to exclude abnormally low thyroid hor mone levels induced by acute or chronic diseases or calorie restrictio n. The subjects were subdivided into three age groups. To avoid condit ions of undernutrition or malnutrition, dietary records were obtained for a sample of 24 subjects, randomly selected and representative of t he whole population for age and sex. 3. In order to properly assess th e influence of selenium status on iodothyronine 5'-deiodinase type I a ctivity, a double-blind placebo-controlled trial was also carried out on 36 elderly subjects, resident at a privately owned nursing home. 4. In the free-living population, a progressive reduction of the T-3/T-4 ratio (due to increased T-4 levels) and of selenium and erythrocyte g lutathione peroxidase activity was observed with advancing age. A high ly significant linear correlation between T-4, T-3/T-4 and selenium wa s observed in the population as a whole (for T-4, R = -0.312, P < 0.00 2; for T-3/T-4 ratio, R = 0,32, P < 0,01) and in older subjects (for T -4, R = -0,40, P < 0,05; for T-3/T-4 ratio, R = 0.54, P < 0.002). 5. T he main result of the double-blind placebo-controlled trial was a sign ificant improvement of selenium indices and a decrease in the T-4 leve l in selenium-treated subjects; serum selenium, erythrocyte glutathion e peroxidase activity and thyroid hormones did not change in placebo-t reated subjects. 6. We concluded that selenium status influences thyro id hormones in the elderly, mainly modulating T-4 levels.