Diagnostics: Radioiodine Uptake Scan

RadioIodine scan

The thyroid, or more specifically the follicular cell of the thyroid, is the predominant type of tissue in the body takes in iodine from the blood. This iodine is used t0 make thyroid hormone. This is the physiological basis for using radioactive iodine in diagnostic scans and therapeutic ablations. The body treats radioactive iodine like any iodine molecule, regardless of whether it is radioactive. Once the radioactive iodine has concentrated to iodine-hungry (“avid”) cells, it emits radiation that may be detected by a scanner (radioiodine uptake scan) or, when higher doses are being used, the higher exposure to radiation can kill the cells that have concentrated it (radioiodine ablation).

Some thyroid cancers also take up iodine. Follicular cells, which give rise to papillary thyroid carcinoma and follicular thyroid carcinoma, normally take up iodine from the blood. When genetic mutations occur such that a normal thyroid follicular cell becomes either a papillary or a follicular thyroid cancer, the cell’s behavior of incorporating iodine often remains intact, as this mechanism is governed by different genes from those that create cancer behavior. As long as a particular strain of papillary or follicular thyroid carcinoma retains the behavior of incorporating iodine, detecting a relatively small cluster of thyroid cancer cells may be accomplished by giving the patient some radioactive iodine, allowing it to circulate and then concentrate in thyroid cells, and then performing a scan to look for concentrated areas of the radioactivity from the radioactive iodine. This technique works best when there is no large remainder of normal thyroid cells left in the thyroid “bed,” (where the thyroid gland is located) after thyroid surgery, because a patch of residual normal thyroid tissue can act like a sponge soaking up most of the radioactive iodine, and leaving little left to collect in any separate focus of thyroid cancer in a lymph node or other location, such as the lung. This is the reason that removing all of the thyroid gland, not just one lobe and the central isthmus, is needed before utilizing radioactive iodine as a method of detection of recurrence.

Only a few other types of tissues take up iodine, including salivary glands, gastric mucosa, mammary glands, and choroid plexus. Knowledge of this by the radiologist reading a radioiodine scan explains why these tissues may collect radioiodine and show up on a scan in these areas as a normal finding.

In the technique described above, the radioactivity in radioactive iodine may be detected by a nuclear medicine scan called a radioiodine uptake scan. In such a diagnostic scan, the amount of radiation exposure may be sufficient to detect a collection of thyroid or thyroid cancer cells, but it is not sufficient to kill these thyroid cells. When radioactive 131-iodine is used in appropriately higher doses, the radiation emitted from the radioactive iodine can kill the thyroid or thyroid cancer cells—a technique called radioiodine ablation therapy.

A thyroid uptake scan can be performed with 131-iodine, 123-iodine, or 99-technetium-labeled pertechnetate. 123-I produces gamma radiation when it decays, which is good for a radioiodine scan. 131-I produces beta radiation (electrons) when it decays, which is good for radioiodine ablation. Using radioactive iodine has an advantage over 99-technetium-labeled pertechnetate because the latter is not iodine, and does not get incorporated into thyroid hormone within the thyroid follicles. Because of this, pertechnate scanning is limited by decreased diagnostic accuracy, as 3-8% of nodules that appear functioning on pertechnetate scanning prove to be nonfunctioning on radioiodine scanning.

Interpretation of radioiodine uptake scans:

Near absent radioiodine uptake

• Subacute lymphocytic (painless/silent) thyroiditis

• Amiodarone-induced thyroiditis

• Subacute granulomatous (painful) thyroiditis

• Iatrogenic or factitious thyrotoxicosis

• Ectopic tissue (S. Ovarii, functional cancer)

• Acute thyroiditis

Normal to increased radioiodine uptake

• Graves’ disease - Radioiodine uptake will be high throughout the entire thyroid gland

• Toxic multinodular goiter

• Toxic solitary nodule - Radioiodine uptake will be strong at the site of the toxic nodule

• Trophoblastic (hCG mediated) disease

• TSH-producing pituitary tumor

• Thyroid hormone resistance