Role of Technetium-99m Pertechnetate in Thyroid Diagnosis
Technetium-99m pertechnetate is primarily used for thyroid imaging to evaluate nodules, differentiate causes of thyrotoxicosis, and detect ectopic thyroid tissue by concentrating in functioning thyroid tissue in a pattern similar to iodine but without being organified.
Mechanism of Action
Technetium-99m pertechnetate functions as a diagnostic radiopharmaceutical that:
- Is trapped by functioning thyroid tissue through the sodium-iodide symporter
- Concentrates in thyroid tissue proportionally to thyroid function
- Does not undergo organification (unlike radioactive iodine)
- Provides visualization of thyroid gland anatomy and function
- Has a short half-life (6 hours) allowing for rapid imaging
FDA-Approved Indications
According to the FDA label, Sodium Pertechnetate Tc-99m is indicated for 1:
- Thyroid imaging in both adults and children
- Salivary gland imaging (adults only)
- Urinary bladder imaging for vesico-ureteral reflux detection
- Nasolacrimal drainage system imaging (adults only)
Clinical Applications in Thyroid Disorders
1. Thyrotoxicosis Evaluation
Technetium-99m pertechnetate is valuable in differentiating between causes of thyrotoxicosis:
- Graves' Disease: Shows diffusely increased uptake throughout the gland
- Toxic Nodular Goiter: Reveals focal areas of increased uptake with suppression of surrounding tissue
- Thyroiditis: Demonstrates decreased uptake due to inflammation-induced thyroid cell damage
Recent research shows that camera-based measurement of Tc-99m pertechnetate uptake has high diagnostic accuracy (sensitivity 93.4%, specificity 94.8%) for differentiating Graves' disease from thyroiditis with a cut-off value of 0.7% 2.
2. Thyroid Nodule Assessment
Technetium-99m pertechnetate helps characterize thyroid nodules:
Functioning ("hot") nodules: Show increased uptake compared to surrounding tissue
- Usually benign (typically adenomas)
- Appear as areas of increased tracer concentration
Non-functioning ("cold") nodules: Show decreased or absent uptake
- Higher risk of malignancy (10-15%)
- Require further evaluation with fine needle aspiration (FNA)
When evaluating thyroid nodules, Tc-99m pertechnetate scanning can be combined with other imaging techniques for improved diagnostic accuracy. Studies show that when used alongside fine-needle aspiration, Tc-99m pertechnetate scanning can help in preoperative assessment of thyroid nodules 3.
3. Congenital Hypothyroidism Screening
Technetium-99m pertechnetate thyroid scans are valuable in evaluating infants with suspected congenital hypothyroidism, allowing characterization of thyroid anatomy into four general types 4:
- Normal size and location
- Ectopic location
- No detectable thyroid activity (agenesis)
- Normal location with increased size or uptake
Technical Considerations
Uptake Measurement
Normal reference ranges for Tc-99m pertechnetate uptake vary by population:
Uptake decreases with age, particularly in females 5
Comparison with Other Imaging Agents
When compared to other thyroid imaging agents:
- Radioactive Iodine (I-123): Tc-99m pertechnetate is more readily available, less expensive, and has a shorter half-life, but doesn't undergo organification
- Tc-99m MIBI and Tc-99m tetrofosmin: These agents can be useful for characterizing thyroid nodules, particularly malignant ones, with comparable findings between tetrofosmin and MIBI studies 6
Clinical Workflow for Thyroid Evaluation
Initial Assessment:
- Ultrasound is the preferred first-line imaging modality for all palpable thyroid nodules 7
- TI-RADS risk stratification system guides management decisions
When to Use Tc-99m Pertechnetate:
- For thyrotoxicosis: To differentiate between Graves' disease and thyroiditis
- For nodular thyroid disease: To determine if nodules are functioning
- For suspected ectopic thyroid tissue: To locate functioning thyroid tissue
Interpretation of Results:
- Increased uptake: Suggests hyperfunctioning tissue (Graves' disease, toxic nodules)
- Decreased uptake: Suggests thyroiditis or non-functioning nodules
- Absent uptake: May indicate thyroid agenesis or complete destruction
Pitfalls and Limitations
- Technetium-99m pertechnetate only reflects trapping function, not organification
- Recent iodine exposure (contrast media, medications) may reduce uptake
- Some medications (e.g., antithyroid drugs) can affect results
- Normal reference ranges vary by population and measurement method
- Limited specificity for malignancy (functioning nodules are rarely malignant, but non-functioning nodules require further evaluation)
By providing visualization of functional thyroid tissue, Technetium-99m pertechnetate scintigraphy remains a valuable diagnostic tool in the evaluation of various thyroid disorders, particularly when used in conjunction with other diagnostic modalities like ultrasound and fine needle aspiration.