When is a thyroid scan recommended?

When is a Thyroid Scan Recommended?

Thyroid scintigraphy (radioiodine uptake scan) is recommended only when TSH is suppressed or low, not in euthyroid patients with thyroid nodules. 1, 2

Primary Indication: Thyrotoxicosis Evaluation

The single most appropriate indication for thyroid scanning is in patients with low TSH to determine the cause of thyrotoxicosis. 1, 2 The scan differentiates between:

• Graves' disease (diffuse increased uptake) 2
• Toxic adenoma (single hyperfunctioning "hot" nodule) 2
• Toxic multinodular goiter (multiple areas of increased uptake) 2
• Thyroiditis (low or absent uptake) 2

For thyrotoxicosis evaluation, three imaging options are considered equivalent first-line choices: thyroid ultrasound, I-123 radionuclide uptake and scan, or I-131 radionuclide uptake with Tc-99m pertechnetate scan. 1, 3 These nuclear medicine studies are complementary with ultrasound. 1

Secondary Indications

Hyperfunctioning Nodule Assessment

When TSH is suppressed and a nodule is present, thyroid scintigraphy identifies whether the nodule is "hot" (hyperfunctioning). 2, 4 Hyperfunctioning nodules are rarely malignant and do not require fine-needle aspiration. 4

Treatment Planning

Thyroid scintigraphy is useful for planning radioactive iodine therapy in patients with hyperthyroidism. 2

Goiter Confirmation

The scan can confirm that an entire goiter mass consists of thyroid tissue, though this is a secondary consideration. 2

When Thyroid Scanning is NOT Recommended

Euthyroid Patients with Nodules

Thyroid scintigraphy should not be performed in euthyroid patients to determine malignancy or decide the need for biopsy. 2 The test has low positive predictive value for malignancy and wastes resources. 2 In euthyroid patients with palpable thyroid nodules, ultrasound is the appropriate first-line imaging. 1, 3

Primary Hypothyroidism

Imaging is not recommended for initial evaluation of primary hypothyroidism. 1 Radionuclide scanning has no role in routine hypothyroidism workup as it does not differentiate causes. 2

Thyroiditis

Radionuclide scanning has no role in routine thyroiditis evaluation unless TSH is suppressed and differentiation between causes of thyrotoxicosis is needed. 2

Medullary Thyroid Cancer

Whole-body scintigraphy has no role in imaging of medullary thyroid cancer (MTC), as MTC cells do not uptake iodine. 1

Algorithmic Approach

1. Check TSH first before selecting any imaging modality 2
2. If TSH is low (suppressed):
   • Perform thyroid ultrasound to evaluate morphology 2
   • Proceed to radioiodine uptake scan to determine cause of thyrotoxicosis 2
   • Use I-123 over I-131 for superior image quality 2
3. If TSH is normal or elevated:
   • Use ultrasound as first-line imaging 1, 3
   • Do NOT proceed to thyroid scan 2

Common Pitfalls to Avoid

• Ordering thyroid scans in euthyroid patients with nodules is inappropriate and exposes patients to unnecessary radiation without diagnostic benefit. 2
• Using radionuclide scanning to screen for thyroid cancer is not recommended, as it cannot distinguish between benign and malignant nodules in euthyroid patients. 2
• Failing to check TSH before imaging leads to inappropriate test selection and wasted resources. 2
• Assuming all nodules need scanning is incorrect; only nodules in patients with suppressed TSH require scintigraphy. 2, 4