But you don't know if you have pre-existing thyroid issues. Prolonged use should include monitoring thyroid function.

see https://www.cdc.gov/healthywater/drinking/travel/backcountry...

Water that has been disinfected with iodine is NOT recommended for pregnant women, people with thyroid problems, those with known hypersensitivity to iodine, or continuous use for more than a few weeks at a time.

Ok, so if clean water is unavailable for more than a few weeks, hop on down to the clinic and trade some ammo for a thyroid checkup? If you're planning for the end of the world, then yes you'll want a backup water plan. For realistic emergency scenarios iodine is a practical water treatment option.