Question

Which radioisotope is used to diagnose thyroid disorders?

Answer 1

Iodine-123.

Explanation:

Iodine is an element that is almost exclusively taken up by the thyroid. In the thyroid, iodine is 'trapped' and bound to an organic molecule. This process is called organification. All vital thyroid cells are able to do this. Iodine is required for the formation of thyroid hormones.

Because of this specificity, a radioactive isotope of iodine can be used to image the thyroid. There are many radioactive isotopes of iodine, for imaging Iodine-123 (I-123) is most often used.

I-123 is a positron (`beta^+`) emitter, therefore positron imaging techniques are used such as PET and SPECT. A positrion will recombine with an electron and emit 2 photons in opposite directions (see image). These photons are detected and form the image of the thyroid.