What is Thyroid Uptake?
Thyroid uptake (radioactive iodine uptake, RAIU) is a nuclear medicine test that measures how much radioactive iodine the thyroid gland absorbs over a specific time period (typically 4,6, or 24 hours), primarily used to determine the cause of hyperthyroidism and calculate radioactive iodine therapy doses. 1
Primary Clinical Applications
Thyroid uptake scanning should only be performed when TSH is suppressed (low), not in euthyroid patients. 2 The test serves two main purposes:
Differentiating causes of thyrotoxicosis: The scan distinguishes between Graves' disease (diffuse high uptake), toxic adenoma (single hot nodule), toxic multinodular goiter (multiple hot areas), and destructive thyroiditis (low uptake). 1, 2, 3
Calculating radioactive iodine therapy doses: The 24-hour uptake value is essential for determining the appropriate therapeutic dose of I-131 for treating hyperthyroidism. 4, 3
Technical Considerations
The test uses radioactive iodine tracers, with I-123 preferred over I-131 due to superior image quality. 2 Key technical points include:
Liquid tracers are more accurate than capsule forms: Diagnostic uptakes performed with liquid I-131 tracers more accurately predict therapeutic uptakes (63% vs. 58% mean values) compared to capsule tracers (44% vs. 58%), raising concerns about capsule bioavailability. 5
Early uptake measurements (4-6 hours) can predict 24-hour values with approximately 7% average dosage error when using appropriate conversion formulas, though patients with early-to-late uptake ratios ≥1 (rapid turnover) require full 24-hour studies. 6
Normal ranges vary geographically and temporally based on dietary iodine intake, with contemporary ranges typically 9-32% (or 11-30% in some populations), significantly lower than historical values of 15-45%. 7
Critical Diagnostic Algorithm
The American College of Radiology recommends starting with TSH measurement before any thyroid imaging. 2 The pathway proceeds as follows:
If TSH is low (suppressed): Perform thyroid ultrasound first to evaluate morphology and nodules, then proceed to radioiodine uptake scan to determine the cause of thyrotoxicosis. 2
If TSH is normal (euthyroid): Thyroid ultrasound is the preferred first-line imaging; uptake scanning is not indicated and wastes resources with low diagnostic value. 2
If TSH is high (hypothyroid): Imaging is generally not indicated; uptake scanning shows decreased uptake in all causes of hypothyroidism and does not differentiate among them. 2
Common Pitfalls to Avoid
Do not use radioiodine uptake scanning to determine malignancy in euthyroid patients with nodules—it has low positive predictive value for cancer and should not replace ultrasound-guided evaluation. 2
Avoid proceeding directly to uptake scan without checking TSH first, as this leads to unnecessary testing and radiation exposure. 2
Be aware that iodinated contrast media can temporarily affect thyroid function and uptake scan accuracy—all radioactive iodine treatment should be deferred for at least 6 weeks after contrast administration. 1
In geographic areas with changing dietary iodine content, the 24-hour RAIU has become a poor diagnostic test for hyperthyroidism, particularly for toxic nodular goiter where 80% of patients may have uptake values within normal limits. 4 Each facility should establish its own normal range based on local dietary iodine factors. 7