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.

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.

When kids have more iodine in their urine, fluoride doesn’t seem to make their thyroid gland grow as much—so iodine might help protect their thyroid from fluoride’s effects.

Boys' thyroids tend to get bigger than girls' when they're exposed to fluoride in water or food, which suggests boys might be more sensitive to fluoride's effects on their thyroid gland.

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.

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.