@ Dolly da Gucci,
I do not think I can give you a comprehensive answer to this question because I do not know. Radioactive atoms have unstable nuclei. It is the lose of energy from these nuclei that causes radioactivity. Radioactive elements either have no stable naturally occurring isotope, or else are entirely artificial. All artificial elements have no stable isotopes.
Some of the uses of radioactivity in biology and medicine fall in a field known as radiology ie Dolly, you can study medicine and if radioactivity fascinates you, you could still specialize in radiology 🙂 . X-rays, computerized tomography (CT) scans, mammography and nuclear medicine are some of the examples where radiologists work. An example of radioactive medicine include the use of radioactive iodine in treatment of thyroid cancer.