tracer

In science, a small quantity of a radioactive isotope (form of an element) used to follow the path of a substance. Certain chemical reactions or physical or biological processes can then be monitored. The location (and possibly concentration) of the tracer is usually detected by using a Geiger–Muller counter.

For example, the activity of the thyroid gland can be monitored by giving the patient an injection containing a small dose of a radioactive isotope of iodine, which is selectively absorbed from the bloodstream by the gland.