Radioactive iodine

Radioactive iodine

A substance used for both diagnostic purposes and therapeutic reasons in suspected or known cases of thyroid disease. The iodine isotopes I-123, I-125, and I-131 can all be used for radioactive iodine uptake studies and scans of the thyroid gland. Typically I-123 is chosen, although technetium pertechnetate is also often used as it is less expensive and more readily available. For uptake purposes, only an iodine isotope can be used.


Radioactive iodine is given orally. It is absorbed from the gut and then trapped by the thyroid
gland. The more active the thyroid is, the more iodine trapped in the gland. Thus, a radioisotope
uptake scan will provide an excellent measure of the physiological activity of the thyroid.

The patient is brought into the nuclear medicine department, and the amount of radioactive activity taken up by the thyroid is counted and reported as a percentage. The typical normal range in the United States is from 15–30 percent. With GRAVES’ DISEASE, the percentages usually range from 40–100 percent. In THYROIDITIS, however, the percentage is usually very low, in the 0–5 percent range.

The amount of iodine that is trapped by the thyroid depends on the diet and the amount of iodine available in the food supply, and it also varies greatly around the world; however, it is fairly consistent throughout the world.

Technetium is more often used to scan the thyroid gland or, in essence, to obtain a picture of the
thyroid. It can also help to determine if a palpable abnormality is a nodule; whether the activity in the thyroid gland is homogenous or heterogeneous; if there is other active thyroid tissue around the thyroid gland or elsewhere; and whether functional and/or nonfunctional hot nodules are present.

Nodules that take up iodine or technetium are known as hot nodules and are rarely malignant.
Nonfunctional or cold nodules may have a 5–15 percent risk of malignancy.

Effects of Radioactive Iodine
The major, long-term effect of radioactive therapy is HYPOTHYROIDISM that necessitates the use of lifelong thyroid hormone therapy. Concern has been raised that the use of radioactive iodine therapy may lead to an exacerbation of Graves’ ophthalmopathy (bulging eyes), although most endocrinologists do not feel this is a significant problem.

When there is concern about the possible worsening of these EYE PROBLEMS, patients are often pretreated with a glucocorticoid medication, such as prednisone, to help minimize the impact.