It's a matter of QUANTITY, people!

You've got different potassium Iodide pills for different applications.
1) 50-100% DV: average supplement (SAFE)

2) >100% DV, < Upper Tolerable: hypothyroidism treatment (MOSTLY SAFE, should not be taken unless hypothyroid, should not be the first Iodine supplement you start with - test for effects at above-standard levels first)

3) >Upper Tolerable (toxic levels): pre-radiation exposure military issue, saturates thyroid with iodine, so that the body, which does not differentiate between regular and radioactive Iodine isotopes, is left unable to absorb any more Iodine for the next day or so, and thus doesn't absorb the radioactive stuff. (VERY DANGEROUS AND UNHEALTHY) Tweet