If a baby is exposed to fluoride while still in the womb or as a young child, it might affect how their brain grows and works, possibly by messing with their thyroid hormones, which could lead to lower IQ, changes in brain shape, or hearing problems.

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.

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.

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.

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.