Because the results of the uptake will determine whether you will need surgery to remove your thyroid glands.

The thyroid scan and a radioactive iodine uptake test (RAIU), also known as a thyroid uptake, are types of nuclear medicine imaging.

It can cause goitre.

Tc99m and I131 arent used in thyroid uptake and scan. Specically I123 is used to evaluate thyroid uptake and is also used to perform a scan mostly in addition to a pinhole collimator. Tc99m can be used just for the purpose of size and shape of thyroid but ultrasound is more specific for that and does not introduce a dose of radiation to the patient.

A thyroid uptake scan wouldn't normally be needed unless it is suspected that thyroid tissue was missed during surgery (sometimes thyroid tissue can be found in other areas of the body), or the reason for the surgery was cancer and the removal of all the tissue is vital.