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.

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 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 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.

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.

Fluoride might stop your thyroid from absorbing enough iodine, which it needs to make hormones—this could make an iodine shortage worse and mess up your metabolism.