The oral administration of radioactive 131I is a standard treatment for thyroid carcinoma. One consideration for this therapy is assuring that other people do not receive significant radiation exposure. In particular, federal and state regulatory authorities stipulate that no individual should receive more than 5 mSv (500 mrem) effective dose-equivalent from the released patient. Patients receiving more than 1.11 GBq (30 mCi) of 131I were traditionally confined as in-patients by regulation until their burdens of radioactivity fell below that level or until the external dose rates were less than 50 microSv (5 mrem) per hour at 1 m. Recent regulatory guidance recommends the use of biological elimination as well as physical decay in calculating the confinement time to keep the effective dose-equivalent to members of the public less than 5 mSv. Analysis of a database of more than 250 administrations of 131I for thyroid cancer shows a median effective half-life of at least 14 h, with substantial variation. Thus, discharge criteria for radiation safety purposes should be calculated on the basis of individual measurements. The release of these patients should not always be as prompt as the guidance indicates. The results also challenge some long-used assumptions regarding iodine excretion in this patient population.