Not true. 100% fatality is assured only if you are receiving whole-body exposures that greatly exceed the LD50/60 for human beings. With any reasonable source term, and even rather pessimistic assumptions for dispersion, diluation, and removal coefficients, you aren't likely to approach that.
What uptake of radioiodine does is increase the risk of developing thyroid-related disorders over the long term. Equating increased risk factors to 100% fatality (or foolishness) is an egregious error. As I stated, from an epidemiological viewpoint, it is desirable to evaluate the risks of doing nothing against those that result from taking ill-advised actions. For example, if an exposed population incurs a risk of one excess fatality from a thyroid disorder over a 30-year period, would that be "better" than exposing them to a risk of perhaps 10 in a million who will have a fatal adverse reaction to taking KI? Has such a study been done? What are the relative risks for a large-scale nuclear accident that results in widespread contamination, versus the risks to the public who, in their fear and panic, start taking doses of KI tablets "just in case", and those that are sensitive to it start dropping for no reason at all?
Buy the potassium iodate pills. Medical Corps is the best. In the meantime, consult your physician and/or do some research if you're worried about an allergic reaction. I'm not.
I agree that one should consult one's physician about this matter. Its a simple test to check for adverse reaction, not unlike an allergy test. Chances are, you'll be okay. Better chances are, as a mamber of the general public, that you'll not have to worry about radioiodine uptake in harmful amounts from any kind of nuclear accident.