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 .
Does selenium supplementation affect thyroid function? Results from a randomized, controlled, double-blinded trial in a Danish population.
OBJECTIVE Selenium is present in the active site of proteins important for thyroid hormone synthesis and metabolism . The objective of this study is to investigate the effect of selenium supplementation in different doses on thyroid function , under conditions of suboptimal dietary selenium intake . DESIGN The Danish PREvention of Cancer by Intervention with SElenium pilot study ( DK-PRECISE ) is a r and omized , double-blinded , placebo-controlled trial . A total of 491 males and females aged 60 - 74 years were r and omized to 100 μg ( n=124 ) , 200 μg ( n=122 ) , or 300 μg ( n=119 ) selenium-enriched yeast or matching yeast-based placebo tablets ( n=126 ) . A total of 361 participants , equally distributed across treatment groups , completed the 5-year intervention period . METHODS Plasma sample s were analyzed for selenium and serum sample s for TSH , free triiodothyronine ( FT3 ) , and free thyroxine ( FT4 ) at baseline , and after 6 months , and 5 years of supplementation . RESULTS Plasma selenium concentrations increased significantly and dose-dependently in treatment groups receiving selenium ( P<0.001 ) . Serum TSH and FT4 concentrations decreased significantly and dose-dependently by 0.066 mIU/l ( P=0.010 ) and 0.11 pmol/l ( P=0.015 ) , respectively , per 100 μg/day increase , with insignificant differences between 6 months and 5 years . No significant effects were found for FT3 and FT3:FT4 ratio . CONCLUSIONS In euthyroid subjects , selenium supplementation minutely and dose-dependently affects thyroid function , when compared with placebo , by decreasing serum TSH and FT4 concentrations . Based on these findings , selenium supplementation is not warranted under conditions of marginal selenium deficiency . However , a role for selenium supplementation in the treatment of autoimmune thyroid diseases is still unresolved
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 .