Radiopharmaceutical
Isotope used in nuclear medicine. In addition to having an appropriate half-life, it must be absorbed by or in some other way become attached to a certain type of tissue, a specific organ or an active disease process.
Some nuclides can be used "as is". A good example is radioactive iodine, which is absorbed directly by the thyroid gland. The thyroid needs iodine to build the hormones it produces and cannot distinguish between radioactive and non-radioactive iodine. Another example is radioactive thallium, which is absorbed directly by the heart muscle and can be used to investigate whether parts of the heart are not functioning normally (indicated by reduced uptake of thallium).
GE Healthcare Glossary