In an adult woman presenting with weight loss, heat intolerance, tremor, palpitations and diffuse goitre, how is a radioactive iodine uptake scan performed to confirm Graves disease and what preparation is required?

Radioactive Iodine Uptake Scan for Graves' Disease Confirmation

Performing the Uptake Scan

In an adult woman with clinical hyperthyroidism and diffuse goiter, a radioactive iodine uptake scan should be performed using I-123 (preferred over I-131 for superior image quality) after confirming suppressed TSH, with the scan demonstrating diffusely increased homogeneous uptake throughout an enlarged thyroid gland—typically showing uptake values >30-35% at 24 hours—which distinguishes Graves' disease from other causes of thyrotoxicosis. 1, 2

Technical Procedure

• The scan uses either I-123 or Tc-99m as the radiotracer, with I-123 preferred because it provides better image quality than I-131. 1, 2

• A 24-hour radioactive iodine uptake (RAIU) measurement is performed, with normal values ranging from 7-30% and Graves' disease typically showing elevated uptake >30-35%. 3, 4

• The scan reveals diffuse thyroid enlargement with intense, homogeneous tracer uptake throughout both lobes, distinguishing Graves' disease from toxic nodular goiter (which shows patchy "hot" areas) or thyroiditis (which shows low uptake). 5, 6

Diagnostic Algorithm Before Scanning

The American College of Radiology mandates checking TSH first—only proceed to uptake scan when TSH is suppressed (low), as this confirms thyrotoxicosis and justifies the radiation exposure. 1

• Perform thyroid ultrasound immediately after confirming suppressed TSH but before the uptake scan, because ultrasound identifies coexisting nodules that may require biopsy for malignancy even in hyperthyroid patients—a critical step that prevents missing thyroid cancer. 1, 7

• Measure free T4 and free T3 to confirm biochemical hyperthyroidism (elevated thyroid hormones with suppressed TSH), as this pattern distinguishes true thyrotoxicosis from subclinical disease. 8

• Consider checking TSH receptor antibodies (TRAbs), as positive TRAbs confirm Graves' disease and may eliminate the need for uptake scanning in straightforward cases. 9

Required Preparation

Medication Adjustments

• Discontinue antithyroid drugs (propylthiouracil or methimazole) 3-4 days before radioiodine administration to allow adequate thyroid uptake for accurate measurement. 2, 8

• Stop levothyroxine for 4-6 weeks prior to scanning in post-thyroidectomy patients being evaluated for thyroid cancer, though this prolonged withdrawal is not needed for initial Graves' disease diagnosis. 2

Dietary and Contrast Restrictions

• Avoid all iodine-containing foods (iodized salt, seafood, dairy products, kelp supplements) for at least 1-2 weeks before the scan, as dietary iodine saturates the thyroid and falsely lowers uptake values. 2, 4

• Avoid iodinated contrast media from CT scans or angiography, as contrast can suppress thyroid uptake for weeks to months and render the scan uninterpretable. 1, 2

• Note that in geographic areas with higher dietary iodine intake, the 24-hour RAIU may fall within "normal" limits in 14% of Graves' disease patients and 80% of toxic nodular goiter patients, reducing the test's diagnostic sensitivity. 3

Clinical Pitfalls to Avoid

• Never skip ultrasound and proceed directly to uptake scan in a patient with suppressed TSH, because ultrasound detects suspicious nodules requiring biopsy—coexisting thyroid cancer occurs even in hyperthyroid patients and will be missed without structural imaging. 1, 7

• Do not order uptake scanning in euthyroid patients (normal TSH) with thyroid nodules, as the scan has low positive predictive value for malignancy and wastes resources while exposing patients to unnecessary radiation. 1

• Recognize that Doppler ultrasound showing increased thyroid blood flow ("thyroid inferno" pattern) can suggest Graves' disease with 95% sensitivity and 90% specificity, but radionuclide uptake remains the gold standard because it directly measures thyroid metabolic activity rather than inferring function from blood flow. 5, 6

• Be aware that severely hyperthyroid patients may require beta-blocker therapy (propranolol or atenolol) to control adrenergic symptoms (tremor, palpitations) during the diagnostic workup, as these adjunctive medications do not interfere with uptake scanning. 9, 8

Planning Radioiodine Therapy

• The 24-hour RAIU measurement is essential for calculating the therapeutic I-131 dose, even when the diagnosis is clinically obvious, because the dose calculation uses the formula: 0.2 mCi per gram of thyroid tissue adjusted for the measured uptake percentage. 2, 4

• Typical therapeutic doses for Graves' disease range from 4-10 mCi for standard cases, with larger doses (up to 70 mCi) required for very large glands or toxic multinodular goiter. 2, 4

• Ultrasound measurements of thyroid volume (obtained before the uptake scan) are combined with the RAIU value to calculate the precise ablative dose, achieving >90% success rates with single-dose therapy when properly calculated. 4, 6