Do fluoride and bromine block iodine?

Yes. Fluoride and bromine are halogens, the same chemical family as iodine, and they're structurally similar enough to compete for the same thyroid receptor, the sodium-iodide symporter. When exposure to these halides is high, whether from tap water, pool water, or certain medications, they can occupy the transporter iodine is meant to use, interfering with thyroid iodine uptake. This is well-established endocrinology, distinct from the separate and more actively debated question of whether fluoride at typical water-fluoridation levels affects IQ.