Selenium Sabotage

Selenium is a trace mineral and antioxidant that’s a popular supplement, but some studies have found that increasing selenium intake to five times the recommended allowance, which is 55 micrograms, led to a modest weight gain in men. A new study shows that the effect likely stems from selenium’s role in thyroid hormone metabolism.1

Selenium promotes the activity of enzymes called deiodinases, which convert the relatively inactive T4 thyroid hormone into the five times more active T3 form. In the study men living in a metabolic ward for four months were given foods either naturally high or low in selenium. During the initial 21 days all the subjects got 47 micrograms a day of selenium, or just below the minimum suggested intake of 55 micrograms. After that the men were divided into two groups, with one group getting only 14 micrograms of selenium a day and the others getting a whopping 297 micrograms. That part of the experiment lasted 99 days.

At the 45-day mark those in the low-selenium group showed a 14 percent increase in T3, while those in the high-selenium group experienced a 23 percent drop. The men in the high-selenium group also showed an increase in thyroid-stimulating hormone (TSH), which is secreted by the pituitary gland in the brain and controls thyroid output. The TSH increased in the high-selenium group because the body was attempting to compensate for the lower T3 levels.

By the 64th day the men in the high-selenium group began to gain weight, while those in the low-selenium group lost it. The authors suggest that the high-selenium group had a subclinical hypothyroid, or low thyroid, which explained the weight gain. Just the opposite occurred in the men in the low-selenium group, who showed increases in blood fat and loss of bodyfat, pointing to hyperthyroidism, or thyroid-gland overactivity. Despite the rise in thyroid activity, none of the men in the low-selenium group lost any lean tissue, or muscle, which often accompanies an elevated thyroid state.

One question not answered in the study was why selenium has these effects on thyroid metabolism. It may be that while a certain amount of selenium activates the deiodinase enzymes, past a certain point the mineral may have a paradoxical, inhibiting, effect. Hydrogen peroxidase is required for thyroid hormone synthesis, and it may be that selenium neutralizes the hydrogen peroxide found in the gland as part of glutathione peroxidase, which would interfere with thyroid hormone production. That effect, by the way, is one of the positive features of free radicals, along with helping immune cells kill invading organisms. It’s a type of ‘smart bomb.’

Does that mean you should limit selenium intake as a means of promoting thyroid activity? The researchers suggest that eventually the body adjusts to the increased selenium intake and that thyroid hormone levels return to normal. I can attest to that, since I’ve taken at least 500 micrograms of selenium daily for more than a decade yet have never shown any low thyroid lab values.

Considering the vital role that selenium plays in preventing the onset of the major killers cancer and cardiovascular disease, it would be sheer folly to limit selenium intake as a means of promoting thyroid activity. On the other hand, since taking more than 1,000 micrograms daily over an extended time can prove toxic, leading to such unpleasant effects as garlic breath and loss of nails and hair, it’s prudent to monitor your doses of this trace mineral. IM

1 Hawkes, W.C., et al. (2003). Dietary selenium intake modulates thyroid hormone and energy metabolism in men. J Nutr. 133:3443-3448.