What does it mean if a thyroid scan shows uptake of radioactive iodine (Radioiodine) by a solitary thyroid nodule in a patient?

What Radioactive Iodine Uptake by a Solitary Thyroid Nodule Means

When a thyroid scan shows radioactive iodine uptake by a solitary thyroid nodule, this indicates a "hot" or hyperfunctioning nodule that is autonomously producing thyroid hormone, which has important implications for both malignancy risk and thyroid function. 1

Clinical Significance of a "Hot" Nodule

Malignancy Risk Assessment

• A hot nodule that takes up radioactive iodine is almost always benign and does not require biopsy for cancer evaluation. 1
• The American College of Radiology states that thyroid scintigraphy has low positive predictive value for malignancy, and hot nodules are not considered suspicious for cancer. 1
• This finding effectively rules out malignancy in that specific nodule, which is a key clinical decision point. 1

Functional Status and Natural History

• A hot nodule represents an autonomous thyroid adenoma—a benign monoclonal tumor that produces thyroid hormone independently of TSH stimulation. 2
• These nodules evolve gradually from "warm" (compensated) adenomas to fully autonomous "hot" nodules that can suppress TSH and cause hyperthyroidism. 2
• The rate of progression to overt thyrotoxicosis is approximately 4% per year and depends on nodule size, iodine intake, and patient age. 2
• Most hot nodules harbor TSH receptor mutations that drive autonomous hormone production. 2

Diagnostic Workflow Context

When This Test Should Be Performed

• Radioiodine uptake scanning should only be performed when TSH is suppressed (low), not in euthyroid patients. 1
• The American College of Radiology recommends checking TSH first, then performing ultrasound, and only proceeding to radioiodine scanning if TSH is low. 1
• In euthyroid patients with normal TSH, radionuclide scanning wastes resources and has low diagnostic value. 1

Critical Pitfall to Avoid

• Never skip ultrasound evaluation before or instead of uptake scanning, as coexisting suspicious nodules requiring biopsy for malignancy may be missed. 3
• The scan only shows functional status, not structural characteristics that might indicate cancer in other nodules. 1

Clinical Implications for Management

If the Patient is Hyperthyroid (Low TSH)

• The hot nodule is the cause of thyrotoxicosis (toxic adenoma). 4, 1
• Treatment options include radioactive iodine therapy, which is highly effective with a 98% success rate and only 6% risk of hypothyroidism when extranodular uptake is prevented. 5
• Radioiodine therapy causes significant nodule volume reduction, with mean 32% reduction at 6 months, most occurring in the first 3 months. 6

If the Patient is Euthyroid (Normal TSH)

• The nodule is a "compensated" autonomous adenoma—producing some hormone but not enough to suppress TSH yet. 2
• The surrounding normal thyroid tissue will still show some uptake on the scan because TSH remains detectable. 2
• Close monitoring is warranted given the 4% annual risk of progression to hyperthyroidism. 2

Scan Interpretation Details

• On radioiodine imaging, the hot nodule concentrates the isotope intensely while suppressing uptake in the surrounding normal thyroid tissue (if TSH is suppressed). 2
• I-123 is preferred over I-131 for diagnostic scanning due to superior image quality. 1
• The scan helps differentiate toxic adenoma from other causes of thyrotoxicosis like Graves disease, toxic multinodular goiter, or thyroiditis. 4, 1