What is a radioactive iodine uptake and scan?

What is a Radioactive Iodine Uptake and Scan?

A radioactive iodine uptake (RAIU) and scan is a nuclear medicine test that measures how much radioactive iodine your thyroid gland absorbs from your bloodstream and creates images showing the distribution of that uptake throughout the gland. 1

How the Test Works

The test involves two components that work together to evaluate thyroid function:

• Radioactive tracer administration: You swallow a small amount of radioactive iodine, typically iodine-123 (I-123), which is preferred over iodine-131 (I-131) because it produces superior image quality with less radiation exposure. 1

• Uptake measurement: A specialized camera measures the percentage of radioactive iodine your thyroid absorbs at specific time points—typically at 2 hours and 24 hours after you take the tracer. 1

• Imaging (scan): The same camera creates pictures showing where the radioactive iodine concentrates in your thyroid, revealing whether uptake is diffuse (spread evenly throughout the gland) or localized to specific nodules. 1, 2

Normal Values and What They Mean

• Normal 24-hour uptake ranges from 7% to 30% in iodine-sufficient populations, though this can vary based on dietary iodine intake in your region. 1, 3

• Graves' disease typically shows uptake greater than 30-35% at 24 hours with diffuse, homogeneous tracer distribution throughout both thyroid lobes. 1

• Low uptake (below normal range) occurs in destructive thyroiditis, where the thyroid is releasing stored hormone but not actively producing new hormone. 1

Primary Clinical Uses

The American College of Radiology identifies specific situations where this test provides critical diagnostic information:

• Determining the cause of hyperthyroidism (low TSH): The scan differentiates Graves' disease (diffuse high uptake) from toxic multinodular goiter (patchy "hot" areas) from thyroiditis (low uptake). 1, 4

• Evaluating hyperfunctioning ("hot") nodules: When TSH is suppressed, the scan identifies whether a nodule is autonomously producing excess thyroid hormone, which changes management decisions. 4

• Planning radioactive iodine therapy: The uptake measurement helps calculate the correct treatment dose for conditions like Graves' disease or toxic nodular goiter. 1, 2

• Confirming thyroid tissue: The scan can verify that a mass in the neck or chest is actually thyroid tissue rather than another type of growth. 4

When This Test Should NOT Be Ordered

The American College of Radiology provides clear guidance on inappropriate uses:

• Never order this test in hypothyroidism (high TSH): All causes of hypothyroidism show decreased uptake, so the test provides no useful diagnostic information. 5

• Do not use in euthyroid patients (normal TSH) with thyroid nodules: The scan has low predictive value for malignancy and does not help decide which nodules need biopsy. 4

• Avoid in routine thyroiditis evaluation: Most thyroiditis cases are diagnosed clinically with laboratory tests alone; imaging is unnecessary unless TSH is suppressed and you need to differentiate causes of thyrotoxicosis. 4

Critical Pre-Test Requirements

Several factors can invalidate your results if not properly managed:

• Confirm suppressed TSH before ordering: The American College of Radiology recommends documenting a low TSH level first, as this confirms thyrotoxicosis and justifies the radiation exposure. 1

• Obtain thyroid ultrasound first: Ultrasound should be performed after confirming low TSH but before the uptake scan to detect coexisting nodules that may require biopsy, preventing missed thyroid malignancy. 1

• Strict iodine avoidance for 1-2 weeks: Patients must avoid iodized salt, seafood, dairy products, kelp supplements, and multivitamins containing iodine, as excess dietary iodine saturates the gland and falsely lowers uptake values. 1

• No iodinated contrast media: Recent CT scans or angiograms with iodinated contrast can suppress thyroid uptake for weeks to months, rendering the scan uninterpretable. 1

Interpretation Patterns

The scan reveals distinct patterns for different thyroid conditions:

• Graves' disease: Diffuse enlargement with intense, homogeneous uptake throughout both lobes, typically with 24-hour uptake exceeding 35%. 1

• Toxic multinodular goiter: Multiple areas of increased ("hot") uptake corresponding to autonomous hyperfunctioning nodules, with suppressed uptake in surrounding normal tissue. 1

• Toxic adenoma: Single focal area of intense uptake with suppression of the remaining thyroid tissue. 2

• Thyroiditis (subacute or silent): Markedly reduced or absent uptake despite elevated thyroid hormone levels, because the inflamed gland is releasing stored hormone but not actively trapping new iodine. 1

Comparison with Alternative Tests

• Doppler ultrasound can show a "thyroid inferno" pattern of increased blood flow with approximately 95% sensitivity and 90% specificity for Graves' disease, but radionuclide uptake remains the gold-standard diagnostic test because it directly measures thyroid metabolic activity rather than inferring it from blood flow. 1

• TSH-receptor antibodies (TRAbs) can confirm Graves' disease when positive, potentially obviating the need for an uptake scan in straightforward cases. 1

Safety Considerations

• Beta-blockers are safe during testing: Severely hyperthyroid patients may receive propranolol or atenolol to control adrenergic symptoms during the diagnostic workup; these medications do not interfere with RAIU measurements. 1

• Radiation exposure is minimal with I-123, making it appropriate for diagnostic purposes when clinically indicated. 1