Chemicals in sunscreens may disrupt thyroid hormones

German researchers looked at rats exposed to the UV-absorbing chemicals 4MBC (also used in some anti-aging creams) and benzophenone 2 (BP2).

They found that after treating the rats with 4MBC for 5 days, the levels of thyroid stimulating hormone were significantly raised (with the other main thyroid hormones T3 was unchanged, while T4 was slightly decreased).

Research leader Professor Josef Köhrle (Charite University, Berlin), said:

This change in thyroid hormone levels is typical of the early stages of hypothyroidism. In addition to changes in hormonal levels, we also saw a significant increase in the weight of the thyroid glands. These results indicate that 4MBC is a potent inhibitor of the pituitary-thyroid system in rats.

The group also screened human cells for the effect of the UV filter benzophenone 2 (BP2), and found that it inactivated a key enzyme involved in thyroid hormone production, but these effects were prevented where there was an adequate amount of iodine in the reaction mixture. They later replicated this effect in rats.

Professor Köhrle commented:

These are initial studies which show that UV-absorbing chemicals in sunscreens have effect on animals, so we need to test these findings in humans before coming to any safety conclusions. We also need to bear in mind that sunscreens have a beneficial effect in protecting against skin cancer, and so the last thing I’d say to anyone just now is to stop using sunscreens, but less extensive direct sun exposure might be better, too!

MBCs have been found to be accumulating in the environment, eg in lakes in Switzerland, and we believe that this is a result of sunscreen use. The work has shown that MBC and BP2 are potent disruptors of the pituitary-thyroid hormonal system in rats. It’s early days, but if the same effect is discovered in humans, then we may have to rethink how we protect children and those with existing thyroid problems or those in iodine-deficient areas from sun exposure.