This information is taken directly from the Website of the American Thyroid Association,

Patients given medical radioactive materials may be detained in antiterrorist security sweeps (December 12, 2002)

Authorities at government buildings and transportation centers are using portable radiation detectors to catch terrorists who have been handling radioactive bomb-making materials. The detectors cannot distinguish illegal radioactive materials from the emissions of radioactive pharmaceuticals used medically. Some patients who have received radioactive tests or treatments are setting off the detectors, and some patients are being detained (1).

American Thyroid Association member Dr. Martin Surks and his colleague Dr. Christoph Buettner have published a letter about this problem in the Journal of the American Medical Association (2). They ask physicians to warn patients that they risk being detained after a radioactive test or treatment. According to the letter, the New York City Police Department's Terrorism Task Force suggests that physicians give treated patients a letter to carry in case they are stopped for setting off a radiation detector. The police might want to verify such a letter.

The American Thyroid Association recommends that when patients are given significant amounts of radionuclides, especially radioactive iodine (I131), they should get a statement from their doctor outlining the:

A model form is linked here. It can be downloaded and should be printed on the physician's letterhead.

(1) Baker, Al: High security trips up some irradiated patients, doctors say. New York Times, December 4, 2002.
(2) Buettner, Christoph, and Surks, Martin I.: Police detainment of a patient following treatment with radioactive iodine (letter). JAMA 288(21), December 4, 2002