Human Health and Disease

The thyroid gland takes up iodine from the blood to make thyroxine.
Radioactive iodine-131 given to a patient is selectively absorbed by thyroid tissue, including thyroid cancer cells. The radiation it emits ($\beta$ particles) destroys these cells while sparing most other tissues.
I-131 is also used to treat hyperthyroidism and to study thyroid function.
$C^{14}$ is used for dating, $U^{238}$ for dating rocks and as nuclear fuel source, and radium for older forms of radiotherapy.
So $I^{131}$ is used in the treatment of thyroid cancer.

Cancer cells have lost the normal controls of division, so they divide rapidly and continuously.
Cells are most sensitive to radiation while they are dividing, because the DNA is being replicated and the chromosomes are condensed; radiation damages the DNA and stops further division.
So radiotherapy kills rapidly dividing cancer cells more than normal cells, most of which divide slowly.
So cancerous cells are easily destroyed by radiation due to rapid cell division.