Residency · Residency · Nuclear Medicine

Thyroid Scintigraphy: Tc-99m Pertechnetate and I-123

Overview

Thyroid scintigraphy evaluates thyroid gland morphology and function using radiotracers that are trapped or organified by thyroid follicular cells. The two primary agents are Tc-99m pertechnetate, which is trapped but not organified, and I-123, which is both trapped and organified. When combined with radioiodine uptake measurement, these studies provide a comprehensive functional assessment of the thyroid gland.

Radiopharmaceuticals

Tc-99m Pertechnetate

Tc-99m pertechnetate is trapped by the sodium-iodide symporter but is not organified within the follicular cell. The typical dose is 5 to 10 mCi administered intravenously, with imaging performed 20 minutes after injection. Its advantages include wide availability, low cost, low radiation dose, and rapid imaging. The main disadvantage is the discordant nodule phenomenon: approximately 1 to 3% of nodules that appear cold on pertechnetate imaging are actually hot on radioiodine scanning, because the nodule traps and organifies iodine but does not trap pertechnetate efficiently.

I-123 Sodium Iodide

I-123 is trapped by the sodium-iodide symporter and organified, meaning it is incorporated into thyroid hormone synthesis. The dose is 200 to 400 microCi administered orally, with imaging at 4 to 6 hours and/or 24 hours. I-123 provides a more accurate functional assessment and eliminates the discordant nodule problem. It also allows concurrent uptake calculation. However, it is more expensive, less readily available, and requires advance ordering.

Radioiodine Uptake (RAIU)

The 24-hour uptake is measured using a thyroid probe. The normal range is 10 to 30% at 24 hours, though this varies by dietary iodine intake and institutional norms. Elevated uptake, typically 40 to 80%, is seen in Graves disease and toxic nodular goiter. Suppressed uptake occurs in thyroiditis (subacute or painless), exogenous thyroid hormone use, and iodine excess.

<image>Normal thyroid scintigraphy with Tc-99m pertechnetate showing homogeneous uptake in both lobes with a visible pyramidal lobe variant</image>

Clinical Indications

Hyperthyroidism Evaluation

The primary role of thyroid scintigraphy is differentiating the causes of thyrotoxicosis. Graves disease produces diffusely increased uptake in an enlarged gland. Toxic multinodular goiter shows heterogeneous uptake with multiple hot and cold areas. A toxic adenoma demonstrates focal intense uptake with suppression of the remaining gland. Subacute thyroiditis (de Quervain) and painless (silent) thyroiditis both show globally decreased or absent uptake. Factitious thyrotoxicosis also produces absent uptake, accompanied by a suppressed thyroglobulin level.

Nodule Characterization

Scintigraphy determines the functional status of palpable or ultrasonographically identified nodules. Hot (hyperfunctioning) nodules carry a very low malignancy risk of less than 1%, and biopsy is generally not needed. Warm (isofunctioning) nodules have a malignancy risk of approximately 5%, similar to the general population. Cold (hypofunctioning) nodules carry a 5 to 15% malignancy risk and require further evaluation with fine-needle aspiration based on ultrasound features.

Other Indications

Additional indications include the identification of ectopic thyroid tissue such as lingual thyroid or substernal goiter, evaluation of thyroglossal duct cysts to confirm that normal thyroid tissue exists before surgical excision, assessment of retrosternal extension, and evaluation of congenital hypothyroidism in neonates to distinguish athyreosis from ectopia and dyshormonogenesis.

Scintigraphic Patterns in Disease

Graves Disease

Graves disease produces a diffusely enlarged gland with markedly increased, homogeneous uptake. The RAIU is elevated, typically ranging from 40 to 80%. The pyramidal lobe is often visible, which suggests gland stimulation. Thyroid-stimulating immunoglobulin (TSI or TRAb) is positive.

Toxic Multinodular Goiter (Plummer Disease)

The gland is enlarged and heterogeneous, with multiple areas of increased and decreased uptake. The overall RAIU may be normal to mildly elevated. Autonomous functioning nodules are present with partial suppression of surrounding tissue. TSH is low, and antibodies are typically negative.

Toxic Adenoma (Autonomous Nodule)

A single focus of intense uptake is seen with complete suppression of the contralateral lobe and the remaining ipsilateral tissue. This correlates with a palpable nodule and a suppressed TSH. The RAIU may be normal or mildly elevated. A TSH stimulation test or administration of exogenous TSH can bring back the suppressed tissue on imaging.

<image>Three scintigraphic patterns of hyperthyroidism: diffusely increased uptake in Graves disease, heterogeneous uptake in toxic multinodular goiter, and focal hot nodule with suppressed background in toxic adenoma</image>

Subacute Thyroiditis

Uptake is markedly reduced or absent throughout the gland. The ESR is elevated, and patients present with thyroid tenderness and a history of recent viral illness. The RAIU is below 5% and often below 1%. The condition is self-limited, progressing through thyrotoxic, hypothyroid, and recovery phases.

Marine-Lenhart Syndrome

Marine-Lenhart syndrome represents Graves disease coexisting with one or more autonomous nodules. The scan shows diffusely increased background uptake with one or more superimposed hot nodules. Both TSH receptor antibodies and autonomous tissue contribute to the hyperthyroidism.

Discordant Nodule

A discordant nodule appears cold on Tc-99m pertechnetate imaging but hot on I-123 or I-131 scanning. This occurs because the nodule traps iodine and organifies it but has impaired pertechnetate trapping. The estimated incidence is 1 to 3% of cold pertechnetate nodules. Recognizing this phenomenon is clinically significant because it may avoid unnecessary biopsy of a functionally autonomous nodule. When there is any doubt, I-123 is the preferred agent for nodule characterization.

Technical Considerations

Patient Preparation

Antithyroid medications such as methimazole and PTU should be discontinued 3 to 7 days before the scan if clinically feasible. Iodine-containing medications, particularly amiodarone, must be avoided, and the effect of amiodarone can last for months. Iodine-rich foods such as seaweed should be avoided for several days. Recent iodinated contrast exposure suppresses uptake for 4 to 8 weeks and should be checked before scheduling. Recent thyroid hormone supplementation suppresses TSH and reduces uptake.

Imaging Technique

A pinhole collimator provides superior spatial resolution for thyroid imaging. The patient is positioned supine with the neck hyperextended. An anterior view is obtained, with oblique views added as needed for substernal or posterior nodules. Marker placement over palpable nodules aids correlation with scintigraphic findings.

<image>Tc-99m pertechnetate thyroid scan showing a cold nodule in the right lobe with corresponding ultrasound demonstrating a solid hypoechoic nodule requiring FNA biopsy</image>

Clinical Pearls

Thyroid scintigraphy is most useful when TSH is suppressed in the setting of hyperthyroidism. It is not indicated for evaluating euthyroid nodules, where ultrasound and FNA are the appropriate tools.

A completely suppressed contralateral lobe accompanying a hot nodule confirms autonomy of that nodule.

In subacute thyroiditis, near-absent uptake distinguishes it from Graves disease despite similar thyrotoxic symptoms.

Amiodarone-induced thyrotoxicosis presents a diagnostic challenge: type 1 (underlying thyroid disease with iodine load) may show some uptake, while type 2 (destructive thyroiditis) shows no uptake. Scintigraphy helps distinguish between these types.

Scintigraphic findings should always be correlated with TSH, free T4, T3, thyroid antibodies, and ultrasound findings for a complete assessment.

In neonates with congenital hypothyroidism, scintigraphy localizes ectopic tissue and guides management decisions.

References

• ACR-SNM-SPR Practice Parameter for Thyroid Scintigraphy and Uptake Measurements.
• Ross, D. S., et al. "ATA Guidelines for Hyperthyroidism Management." Thyroid, 2016.
• Intenzo, C. M., et al. "Thyroid Scintigraphy in Clinical Practice." RadioGraphics, 2003.
• SNMMI Procedure Standard for Thyroid Uptake and Scan.

Read this lecture as Markdown