Comparison guide

Selenium or selenomethionine?

Selenomethionine is the organic form found in food; trials used it, selenised yeast or sodium selenite, typically at 200 mcg/day. The bigger question is whether you need any: most Western diets already supply 80–150 mcg/day against an RDA of 55 mcg, and the upper limit is only 400 mcg/day.

The options, compared

Option	Evidence	Best for	The honest note
Selenium	Limited evidence	Correcting genuine deficiency only	Large trials found no cancer prevention, and in people who are already replete it slightly raises type 2 diabetes risk. True deficiency is confined to specific low-selenium regions, severe malabsorption and long-term parenteral nutrition.	View
Selenomethionine	Moderate evidence	Hashimoto's, with medical input	The form used in most trials, including SELECT, which found no cancer prevention. It lowers thyroid peroxidase antibodies in Hashimoto's with moderate certainty, but no trial has shown that this improves clinical outcomes.	View

Our verdict

If you have a reason to supplement, selenomethionine at 100–200 µg/day is the form most trials used. Most people eating a normal Western diet should skip selenium entirely, and nobody should stack it with Brazil nuts or a multivitamin that already contains it, because the margin between enough and too much is narrow.