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Selenium
Selenium is the one supplement where the bottle a friend hands you "for your thyroid" can do more harm than good. Your body needs only about 55 micrograms a day to build the proteins that switch on thyroid hormone and clear cellular waste, and a varied diet covers it. The surprise: the standalone 200 ยตg pill sits in the dose range that raised type 2 diabetes and nudged prostate cancer in trials Stranges et al. 2007. For most readers, confirm you have enough and skip the pill.
Do ยท As-needed Evidence Emerging Chapter Supplements

There is no "more is better" here. Selenium only works built into specific proteins, and that demand has a ceiling: function plateaus around 110 micrograms a day 1. Across 13,887 US adults, both low and high blood selenium track higher mortality, safest in the middle 2. A U-curve, where the harm sits at the top end.

The cancer story collapsed. A 1996 result hinted selenium halved cancer risk; the 35,533-man SELECT trial found no prevention and a worrying uptick in prostate cancer 3. Cochrane's verdict: no cancer prevention in well-fed people 4.

Where it earns its keep is autoimmune thyroid disease. In pregnant women with thyroid antibodies, 200 ยตg/day cut postpartum thyroid dysfunction from 49% to 29% 5. It also improves mild Graves' eye disease 6 and lowers antibodies in Hashimoto's 7 โ€” always in an endocrinologist's hands 8.

Only a few people should act on this. Autoimmune thyroid disease under a specialist; residents of low-soil regions like the UK, where intake has fallen near 40 ยตg/day, or pre-fortification New Zealand 9; strict vegans and people with severe gut malabsorption; and subfertile men whose workup flags low selenium. Anyone eating seafood, eggs, or whole grains a few times a week is already in the safe middle.

First, find out whether you even need it. In the US, Canada, or much of South America the soil is selenium-replete and food covers you. In the UK, much of Europe, or New Zealand, a small daily food source is reasonable.

If a clinician does prescribe it for thyroid disease, the trial-backed dose is 100 to 200 ยตg/day for a bounded course, usually six months, then reassess 5. Higher only adds toxicity risk, since function saturates near 125 ยตg 1.

The reward runs in two directions. If you were truly deficient, blood selenium rises into the functional range within two months 1; a Hashimoto's patient may see antibodies fall on a three-to-six-month panel 7; a treated pregnancy halves its odds of postpartum thyroid trouble about a year out 5. For the replete reader the payoff is quieter: you never trip the diabetes signal and stay in the safe stretch of the U-curve 2. Reach adequacy, then leave it alone.

The fine print โ€” when to skip it, and what people get wrong

"It's an antioxidant, so more helps." Extra selenium past ~110 ยตg does no added work; it only adds toxicity risk 1. "Brazil nuts are safe." Content varies tenfold by region; one nut can top 250 ยตg, and a daily handful has caused selenosis 9.

Don't stack a 200 ยตg pill on a multivitamin that already has selenium โ€” that crosses into the diabetes-risk range 10. The ceiling is 400 ยตg/day in the US, 300 in Europe 11. Don't self-prescribe for vague fatigue or a normal TSH 8.

Overshoot shows as hair breaking off in clumps, cracked nails, and garlic breath; in 2008 a misformulated supplement left 201 people with damage that lasted months 12. Recovery, not another pill, is the fix.

References
  1. 1Combs GF Jr (2015). Biomarkers of selenium status. Nutrients. link
  2. 2Bleys et al. (2008). Serum selenium levels and all-cause, cancer, and cardiovascular mortality among US adults. Archives of Internal Medicine. link
  3. 3Lippman et al. (2009). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. link
  4. 4Vinceti et al. (2018). Selenium for preventing cancer. Cochrane Database of Systematic Reviews. link
  5. 5Negro et al. (2007). The influence of selenium supplementation on postpartum thyroid status in pregnant women with thyroid peroxidase autoantibodies. Journal of Clinical Endocrinology & Metabolism. link
  6. 6Marcocci et al. (2011). Selenium and the course of mild Graves' orbitopathy. New England Journal of Medicine. link
  7. 7Toulis et al. (2010). Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and a meta-analysis. Thyroid. link
  8. 8Winther et al. (2020). Selenium in thyroid disorders โ€” essential knowledge for clinicians. Nature Reviews Endocrinology. link
  9. 9Rayman MP (2012). Selenium and human health. Lancet. link
  10. 10Stranges et al. (2007). Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Annals of Internal Medicine. link
  11. 11Institute of Medicine (2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. link
  12. 12MacFarquhar et al. (2010). Acute selenium toxicity associated with a dietary supplement. Archives of Internal Medicine. link
Related in the handbook (7)
Works well with
  • โ€” Organ meats are one of the richest whole-food selenium sources, easy to overshoot if you eat them often.
Related
  • โ€” Iodine needs adequate selenium to be used properly in the thyroid; the two are obligate partners.
  • โ€” Iodine is selenium's obligate partner in making thyroid hormone; a gap in either reshapes thyroid output.
  • โ€” People with Hashimoto's are among the few who genuinely benefit from paying attention to selenium.
  • โ€” If your thyroid screen turns up Hashimoto's antibodies, selenium is one of the few supplements with real (if modest) evidence behind it.
  • โ€” Vitamin E is the fat-soluble peroxide-cleanup partner that was paired with selenium in the big SELECT trial.
  • โ€” Zinc is synergistic with selenium for immune function and thyroid hormone signalling.
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