In telling them that, you would already alert them to the fact that you have a medical condition, though.
Yes, but there is a big difference between "I have a medical condition that requires XYZ", and "I am an ABC patient and require treatment with XYZ". It's one thing to state you have a medical condition and another to divulge it. I think that's exactly what many people object to at TSA checkpoints - namely that nine times out of ten, if you declare something as being necessary for a medical condition, they shoot back with ""What is your medical conditon?"
The better question is why they would pursue the matter if they are able to isolate the source of radiation as a person's body and the amount of radiation detected is minor.
Is that U-235 in your pocket or . . .
I would have thought following up with a well worded generic question would tweeze out whether or not someone had received a radioactive pharmaceutical. Leave the theatrics and good cop bad cop to Hollywood please, and let passengers negotiate an already needless exercise (immigration and customs) with the dignity they deserve.