I think KI tablets just reduces the rate at which you absorb rads in game. In real life, you are supposed to take them before you are exposed, to prevent your thyroid from absorbing radioactive Iodine-131, which is found in nuclear fallout. It basically fills up your thyroid so there is no room for the I-131, which you then piss out. You will still have absorbed a dose of radiation, but it won’t be sitting there continuously irradiating you. Being overburdened DOES damage you if you are sufficiently overburdened, and will cause a gradual buildup of pain.