The RAI-Uptake Test and Scan
by Elaine Moore
September 3, 2000Patients who are initially diagnosed with hyperthyroidism are often ordered to have a radioiodine uptake test (RAI-U) or scan. The idea here is that true hyperthyroidism can be distinguished from conditions where patients happen to be taking excess thyroid hormone or have transient thyroiditis due to infections.
Most importantly, the docs want to know if the patient has Graves' disease, or autoimmune hyperthyroidism, which is responsible for more than 90% of all cases of hyperthyroidism in the West. Why is the RAI-U no longer needed? A blood test for stimulating TSH receptor antibodies is more specific and it doesn't expose the patient and his or her already stressed thyroid to excess radioiodine.
Because the RAI-U is seldom properly explained, many hyperthyroid patients are confused and think they're receiving radioiodine ablation to destroy their thyroids. The RAI-U has a similar principle but a smaller amount of a different isotope is used for the diagnostic test. For ablation, I-131 is usually used, and for the RAI-U, I-125 is used.
For both the test and the ablative procedure, radioiodine is given orally as a dose or drink. In the test, however, the amount of radioiodine taken up by the thyroid gland is measured by an imaging test after any period of time from 2-24 hours. At the same time, a scan is done to show how the iodine is distributed throughout the gland. In Graves' disease, the uptake is high and the pattern of the scan is diffuse. In the case of nodules, there will be spots in the scan with differences in density.
The normal range for the RAI-U is 8% to 35% although the ranges are different in various geographic locations, relative to dietary iodine concentrations. Some researchers say that with the increased iodine content of the American diet, the RAI-U no longer shows clear abnormalities.
RAI-U is increased in hyperthyroidism, iodine deficiency, pregnancy, hydatidiform mole, recovery phase of subacute thyroiditis, rebound after TSH suppression, rebound phase after withdrawal of strong iodine solution or anti-thyroid drugs if the TSH is elevated, therapeutic lithium and inborn errors of thyroid hormonogenesis. From this list, it's clear that the RAI-U is not a specific test for diagnosing Graves' disease. In fact, in patients with T3 thyrotoxicosis, the value may be decreased. The test for stimulating TSH receptor antibodies or TSI is diagnostic for Graves' disease.
There are several sources of error for the RAI-U, most significantly iodine contrast dyes used in imaging studies. These may interfere with test results for several months. Excess dietary iodine in the form of kelp or as inorganic iodine in multivitamins also skewers the results.
And according to John Gofman, a physician and doctor on nuclear/physical chemistry, the radioiodines in diagnostic procedures are just as hazardous as the radioiodine used as an ablative treatment. Gofman cautions that the effects of ionizing radiation on chromosomes in stem cells may take more than 30 years to emerge.
Most docs prefer the RAI-U test. After all it's done locally (often at the clinic at the doctor's office, generating revenue for the clinic) and results are generated quickly, compared to the blood test which usually needs to be sent to a national reference lab and takes several days for results. However, the blood test is superior for diagnosing Graves' disease as well as a safer approach.
Thyroid Home
Back to List of Thyroid Articles