Read Dr. David Brownstein’s boo, Iodine Deficiency or watch his many podcasts appearances. 50% of the population is iodine deficient. While deficient, the body takes up more fluorine and bromine (potassium bromide is common ingredient in U.S. bread).
This messes up the body in several ways: leach into bones to make them more brittle, greatly increase the chances of prostate, breast, and ovary cancers, cause hypothyroidism, and seams to cause the energy swings of type 2 bipolar because that’s thyroid/pituitary based. It seems to disrupt how cells produce energy, but I know very little about how.
Fluoride never leaves the body. If you consume enough iodine, it leaches into the bones, you do not expel it. It’s only good for enamel but no where else, so brush with it if you want and spit it out.
The cheapest way to supplement with iodine is with Lugol’s solution. It’s a 50/50 solution of potassium iodide and iodine. It’s cheap on Amazon. I use a few drops every day in electrolytes and I never get hang overs or headaches now. That was nice to find out.