Although observational evidence suggests that concentrations of selenium , zinc , and iron are positively associated with iodine status , data from r and omized controlled trials fail to confirm this relationship .
Context The metabolism of thyroid hormones , which are essential for normal development , involves many proteins and enzymes .
It requires iodine as a key component but is also influenced by several other micronutrients , including selenium , zinc , iron , and vitamin A. Objective This systematic review was design ed to investigate the effect of micronutrient status and supplementation on  iodine status and thyroid hormone concentrations .
Low selenium status in the elderly influences thyroid hormones.
1 . Iodothyronine 5'-deiodinase , which is mainly responsible for peripheral triiodothyronine ( T3 ) production , has recently been demonstrated to be a selenium-containing enzyme . In the elderly , reduced peripheral conversion of thyroxine ( T4 ) to T3 and overt hypothyroidism are frequently observed . 2 . We measured  serum selenium and erythrocyte glutathione peroxidase ( as indices of selenium status ) , thyroid hormones and thyroid-stimulating hormone in 109 healthy euthyroid subjects ( 52 women , 57 men ) , carefully selected to exclude abnormally low thyroid hormone levels induced by acute or chronic diseases or calorie restriction . The subjects were subdivided into three age groups . To avoid conditions of under-nutrition or malnutrition , dietary records were obtained for a sample of 24 subjects , r and omly selected and representative of the whole population for age and sex . 3 . In order to properly assess the influence of selenium status on iodothyronine 5'-deiodinase type I activity , 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 T3/T4 ratio ( due to increased T4 levels ) and of selenium and erythrocyte glutathione peroxidase activity was observed with advancing age . A highly significant linear correlation between T4 , T3/T4 and selenium was observed in the population as a whole ( for T4 , R = -0.312 , P < 0.002 ; for T3/T4 ratio , R = 0.32 , P < 0.01 ) and in older subjects ( for T4 , R = -0.40 , P < 0.05 ; for T3/T4 ratio , R = 0.54 , P < 0.002 ) . 5 . The main result of the double-blind placebo-controlled trial was a significant improvement of selenium indices and a decrease in the T4 level in selenium-treated subjects ; serum selenium , erythrocyte glutathione peroxidase activity and thyroid hormones did not change in placebo-treated subjects . 6 . We concluded that selenium status influences thyroid hormones in the elderly , mainly modulating T4 levels
Although observational evidence suggests that concentrations of selenium , zinc , and iron are positively associated with iodine status , data from r and omized controlled trials fail to confirm this relationship .
Context The metabolism of thyroid hormones , which are essential for normal development , involves many proteins and enzymes .
It requires iodine as a key component but is also influenced by several other micronutrients , including selenium , zinc , iron , and vitamin A. Objective This systematic review was design ed to investigate the effect of micronutrient status and supplementation on  iodine status and thyroid hormone concentrations .