We follow a radioactive sample from production in a reactor to its use in a biological application to model the complexities in the use of radioactive isotopes from a student's perspective. Specifically, we describe a way to use gamma ray detection outside the body to estimate the clearance rate of these radioisotopes from the body and how that rate can be used to assess thyroid cancer remnant mass. This procedure has a long history in nuclear medicine and is well established. Classroom discussion questions are included to think more deeply about the proper (and improper) handling of radiopharmaceuticals as well as patient safety.