Selenium Supplements Guide (2026): Thyroid Evidence, Dose & the Narrow Safe Range

Selenium is a trace mineral concentrated in the thyroid, where it powers the enzymes that convert and protect thyroid hormone. Its best evidence is in Hashimoto's thyroiditis: several trials of 200 mcg/day (usually selenomethionine) show reduced TPO antibodies, though whether that changes how people feel is less certain. The critical caveat is safety — selenium has one of the narrowest safe ranges of any nutrient. The tolerable upper limit is 400 mcg/day, and chronic excess causes hair loss, brittle nails, and nerve problems. More is not better. Below: the evidence and honest dosing.