Influence of high dietary selenium intake on the thyroid hormone level in human serum

Effects of high dietary selenium supply (range 170-980 micrograms per day) on the metabolism of thyroid hormones were studied in serum of mothers living in seleniferous areas of Venezuela. Free thyroxine (FT4), free triiodothyronine (FT3) and human thyroid stimulating hormone (hTSH) were found to be within the normal range but a significant inverse correlation was found between the FT3 and selenium. It was hypothesized that the activity of hepatic selenoenzyme type I iodothyronine 5'-deiodinase, which catalyzes the production of T3 from T4, becomes depressed at high levels of dietary intake of selenium. The effect is discussed with respect to the safe level of dietary selenium intake, which was estimated to be below 500 micrograms per day.