Exposing the thyroid to ionizing radiation is pretty serious especially in children but it all depends on the amount of radiation you are exposed to. See the thyroid collars and lead aprons below.
Your off on the skin contact chemical. I don't think what Danny Gordon was referring to was like the Sarin attack in Japan. In the Danny Gordon case the skin contact chemical was supposed to be lethal. Not a non lethal weapon nor a chemical attack where a person is being gassed.
Still, how is the weapon effective against those without a thyroid?
Lethal or not does not change the nature of such highly transmittable substances? That is why they are so effective.