After a 123 I thyroid study, when is it appropriate to perform a 125 I plasma volume measurement?

In the context of performing a 125I plasma volume measurement after conducting a 123I thyroid study, it is important to understand the properties of these isotopes. The correct choice indicates that the 123I will not interfere with the subsequent measurement of 125I.

123I and 125I are both iodine isotopes used in nuclear medicine, but they have different physical and chemical properties. When a patient undergoes a 123I thyroid scan, the amount of radioactive iodine administered is relatively small, and its short half-life (approximately 13 hours) means that it will decay quickly. By the time a plasma volume measurement with 125I is performed, the level of 123I in the bloodstream will have decreased to a point where it won't significantly impact the results of the 125I measurement.

This lack of interference allows for an effective transition from one isotope measurement to another without needing to wait for the 123I to fully decay, which can be time-consuming and inconvenient. Therefore, measuring plasma volume with 125I shortly after a 123I study is considered appropriate since the two isotopes can coexist in the body without significant cross-interference.

The other options are less optimal: postponing the test until the 123I has