People who drink water with a lot of fluoride for a long time may have lower levels of certain thyroid hormones and higher levels of a hormone that tells the thyroid to work harder—this could mean their thyroid isn’t working quite right.

Your thyroid gland might soak up more fluoride than other parts of your body, which could make it more likely to get harmed by it.

Too much fluoride might mess up the thyroid’s ability to handle harmful chemicals in the body, which could hurt the cells that make thyroid hormones and make the gland work less well.

When kids have just enough iodine in their urine but are exposed to more fluoride, their bodies might make less of an important thyroid hormone called TT3 — like their thyroid is struggling a bit under those conditions.

In kids, more fluoride in their urine seems to be linked to bigger thyroids, but iodine in their urine doesn’t seem to make a clear difference—so fluoride might be the main thing affecting their thyroid size.

In kids aged 7 to 12, the more fluoride they have in their urine, the bigger their thyroid gland tends to be — by a tiny amount, about the size of a small grape.