A while back I wrote an article on the role of selenium in autoimmune thyroid disease.
I summarized several studies, which found that selenium supplementation reduced inflammation and damage to thyroid tissue, lowered thyroid antibody levels in the blood, and improved the conversion of T4 to T3.
However, I also sounded a note of caution when it comes to selenium supplementation:
“These preliminary studies show the positive effects of selenium supplementation on inflammatory activity in autoimmune thyroid conditions, but the long-term effects of supplementation on thyroid health are still unknown. And we know that selenium is an essential component of the enzymes that convert T4 to T3, but whether supplementation will increase serum T3 levels is unclear.
While it seems that selenium supplementation would be an obvious solution to poor thyroid function, long-term consumption of high doses of selenium can lead to complications such as gastrointestinal upsets, hair loss, white blotchy nails, garlic breath odor, fatigue, irritability, and mild nerve damage. (1) Additionally, supplementing with selenium in the context of low iodine status may actually aggravate hypothyroidism. Mario Renato Iwakura discusses this particular topic extensively on Paul Jaminet’s Perfect Health Diet blog.
For now, the best option for most people may be to include selenium-rich foods in the context of a healthy Paleo diet.”
A study published last year—which I just came across a few weeks ago—appears to validate this caution, at least for middle-aged men. (2) It was a large clinical trial (with over 35,000 participants) that examined the relationship between baseline selenium status, supplementation with selenium and vitamin E (either together or separately), and future rates of prostate cancer in men over 55 years of age.
Here’s what the researchers found:
- Baseline selenium status alone was not associated with prostate cancer risk.
- In men with high baseline toenail selenium (>60th percentile), selenium supplementation of 200 mcg/d increased the risk of high-grade prostate cancer by 62% when taken alone, and by 224% when taken with vitamin E.
- In men with normal or low baseline toenail selenium (<60th percentile), selenium supplementation of 200 mcg/d (either alone or with vitamin E) did not substantially increase or decrease the risk of prostate cancer.
It’s worth pointing out that these findings contradict two previous (and considerably smaller) observational studies in the United States, which found that low toenail selenium status increases the risk of prostate cancer. (3, 4) In addition, a large Dutch study including over 58,000 participants found that those with the highest toenail selenium status had a 37% lower risk of prostate cancer than those with the lowest selenium status. (5)
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However, in the United States, selenium levels are typically much higher than they are in Holland, where deficiency is widespread. While it’s entirely possible that the very low selenium levels seen in Holland are associated with increased prostate cancer risk, there does not appear to be a correlation between selenium and prostate cancer in the United States—at least within the ranges of toenail selenium found in U.S. men over 55 years of age.
So what are we to make of this?
But if baseline levels are normal or high, supplementation may cause harm—as it appears to have done in this study. This shouldn’t be too much of a surprise; most nutrients have a “U-shaped curve”, which means that both too little and too much can cause problems.
On the other hand, several studies indicate that supplementing with selenium has several benefits for people with autoimmune thyroid disease, including reduced inflammation and damage to thyroid tissue. At least one study found that selenium supplementation produced these benefits even when selenium levels were normal at baseline. (7) Since both Hashimoto’s and Graves’ disease are associated with increased morbidity and mortality, an intervention that reduces the autoimmune/inflammatory burden in these conditions might be expected to reduce morbidity and mortality.
There’s little doubt that maintaining adequate selenium levels is important to immune and thyroid function. But given the potential long-term risk of supplementation, I think the best option for most people is to meet their need for selenium by eating selenium-rich foods. Great sources include: brazil nuts, crimini mushrooms, cod, shrimp, tuna, halibut, salmon, scallops, chicken, eggs, shiitake mushrooms, lamb, and turkey. Brazil nuts are a particularly rich source; just 2–3 a day will provide roughly 200 mcg of selenium, which is the amount used in many of the autoimmune thyroid studies.
Finally, while this study was focused on prostate cancer risk in men over 55, it’s probably prudent to assume that long-term selenium supplementation in both younger men and women of any age may also have undesirable effects.
I will, of course, continue to keep an eye on the research in this area and report back if any new findings come to light that would change my recommendation.
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