radioactive iodine
Meaning of radioactive iodine: Iodine carrying a radioactive label, taken up by thyroid cells so that it can either photograph them or destroy them.
Definition of radioactive iodine
Radioactive iodine is a form of iodine whose atoms give off radiation. Thyroid cells pull iodine out of the blood to build thyroid hormone and cannot tell the radioactive kind apart, so a swallowed dose collects in thyroid tissue wherever it sits in the body. A small dose reveals that tissue on a scan, and a large dose destroys it from within.
Why does it go straight to the thyroid?
The thyroid is the only tissue that concentrates iodine strongly, using a pump in its cell membranes to draw it from the bloodstream. That single fact is what makes the treatment possible. Radiation is delivered to the target and largely spares everything else, without surgery and without an external beam. Thyroid cancer cells usually keep the pump, so deposits that have spread to lymph nodes, lung or bone take up the dose as well.
What is it used for?
| Purpose | What happens |
|---|---|
| Overactive thyroid | A measured dose reduces hormone output over weeks to months |
| Thyroid cancer after surgery | Destroys remaining thyroid tissue and any deposits elsewhere |
| Whole-body scan | A tiny dose maps where thyroid tissue remains after treatment |
| Uptake test | Measures how greedily the gland takes iodine, separating causes of overactivity |
Does the isotope matter?
It decides everything. Iodine-131 gives off both beta particles, which travel a fraction of a millimeter and do the destroying, and gamma rays, which escape the body and can be photographed. Iodine-123 gives off gamma rays only, so it images without damaging tissue and is preferred when a picture is all that is wanted. Other iodine isotopes are sealed inside tiny metal radioactive seeds and placed inside the body as a form of internal radiation therapy, a different use of the same element.
What surrounds a treatment dose?
For several days afterward the person emits low-level radiation, so contact with others is limited for a defined period and simple household precautions are followed. The salivary glands also take up iodine and may become sore or dry, and taste can change for a while. Treatment is not given during pregnancy or breastfeeding. Treating an overactive gland very often leaves it underactive, which is expected rather than a complication and is handled with hormone replacement.
Also known as
Used in a sentence
Radioactive iodine was given after thyroidectomy to ablate the remaining thyroid tissue.
Good to know
Iodine-131 both treats and can be imaged while iodine-123 only images, and treating an overactive thyroid commonly leaves the gland underactive, which is an expected outcome rather than a complication.
Word origin
Where “radioactive iodine” comes from
Iodine comes from the Greek iodes, meaning violet colored, after the purple vapor the element gives off when heated.