Fears have been raised about the potential of sunscreens to disrupt the balance of thyroid hormones.
A chemical in sunscreens may alter thyroid hormones
German researchers found rats exposed to the sunscreen chemical 4MBC had raised levels of thyroid stimulating hormones and heavier thyroid glands.
The gland sits in the neck and produces hormones to regulate metabolism.
If thyroid levels are altered it can cause lethargy, but Charite University said more research was needed to see if this also happened in humans.
Another chemical benzophenone 2 was also found to alter thyroid hormone levels, but the effect was reversed by other chemicals present in sunscreens.
One in 50 people - mainly women - are affected by altered hormone levels.
Lead researcher Professor Josef Kohrle said more research was needed to test the findings, which were presented to the European Congress of Endocrinology.
"These are initial studies which show that UV(ultra-violet light)-absorbing chemicals in sunscreens have an effect on animals, so we need to test these findings in humans before coming to too many 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."
Dr Prakash Abraham, of the British Thyroid Association, said the findings were interesting but agreed people should not be put off using sunscreens just yet.
"I have not heard of this happening before, more research is needed."