Thyroid Scan (Radionuclide Scintigraphy) - 10 Marks

Introduction

Radiotracers Used

1. Technetium-99m Pertechnetate (99mTc)

• Most commonly used isotope for thyroid scanning
• Emits primarily gamma (γ) radiation; short half-life (~6 hours)
• Taken up by thyroid tissue like iodine but not organified (not incorporated into thyroglobulin)
• Imaging must be performed within 20-30 minutes of intravenous injection
• Lower radiation dose to the patient

2. Iodine-123 (123I)

• Emits x-rays, β particles, and γ rays
• Images obtained at 4 and 24 hours after oral administration
• Allows calculation of 24-hour radioactive iodine uptake (RAIU)
• Normal 24-hour uptake: 10-35%
• Preferred when measuring functional uptake over time

3. Iodine-131 (131I)

• Longer half-life (~8 days); emits more β and γ radiation
• Cleared thoroughly from background tissues
• Used for diagnostic imaging (1-5 mCi) and therapeutic ablation (30-150 mCi) of residual thyroid tissue after thyroidectomy for differentiated thyroid cancer
• Isotope of choice for imaging differentiated thyroid cancers and their metastases

Principle

• "Hot" areas - increased radiotracer uptake = hyperfunctioning tissue
• "Cold" areas - decreased/absent radiotracer uptake = hypofunctioning tissue

Indications

As emphasized in Bailey & Love's Surgery: thyrotoxicosis with a nodule is one of the few indications for a radioiodine uptake scan in thyroid neoplasm workup.

Scintigraphic Patterns and Their Interpretation

1. Diffuse Increased Uptake (Hot Gland)

• Entire gland shows uniform increased uptake
• Seen in: Graves' disease - TSH-receptor antibody stimulation causes diffuse hyperfunction
• In Graves' disease, uptake remains elevated at 24 hours

2. Focal Hot Nodule with Suppressed Background

• Single area of increased uptake with suppression of normal tissue
• Toxic (autonomous) adenoma - nodule functions independently of TSH
• Hot nodules are very rarely malignant (<1%)

3. Multiple Hot Areas with Heterogeneous Background

• Toxic multinodular goitre (TMNG) - multiple autonomously functioning nodules

4. Diffuse Low / No Uptake

• Seen in:
  • Transient (subacute) thyroiditis - release of preformed thyroid hormones, not new synthesis
  • Iodine-induced hyperthyroidism (Jod-Basedow)
  • Factitious thyrotoxicosis (exogenous thyroid hormone ingestion)
  • Hashimoto's thyroiditis in later stages

5. Cold Nodule

• Single area of reduced/absent uptake within the gland
• Associated with higher risk of malignancy (historically considered suspicious)
• However, in modern practice, FNAC biopsy has largely replaced scintigraphy for malignancy assessment - cold nodules must still be evaluated with ultrasound and FNAC

Procedure

1. Patient preparation: stop antithyroid drugs, iodine-containing medications, and contrast agents for an appropriate period
2. For 99mTc: intravenous injection, imaging at 20-30 minutes
3. For 123I: oral capsule/liquid, imaging at 4 and 24 hours
4. Gamma camera images the neck; the patient lies supine with the neck extended
5. Computer analysis generates a functional map of the thyroid

Limitations

• Provides functional information only - poor anatomical detail compared to ultrasound
• Lacks specificity - high RAIU values can occur in normal glands and iodine deficiency
• Cannot reliably detect multigland disease (especially parathyroid pathology without subtraction)
• Cold nodule classification is not specific for malignancy; FNA has replaced it as the diagnostic gold standard
• Radiation exposure (though low for 99mTc)
• Contraindicated in pregnancy and breastfeeding (especially 131I)
• Results affected by recent iodinated contrast CT (delay scan by at least 3 weeks after contrast administration)

Subtraction Thyroid Scan

• Patient receives sestamibi (accumulates in both thyroid and parathyroid)
• Followed by a thyroid-specific tracer (123I or 99mTc pertechnetate)
• Computer software subtracts the thyroid image from the combined image, revealing parathyroid tissue
• Limitation: image quality can be compromised by motion artefact

Comparison of Isotopes

Clinical Relevance in ENT/Head and Neck Surgery

• Guides decision-making between surgery, radioiodine therapy, and antithyroid drugs in hyperthyroidism
• Identifies autonomous functioning nodules that can be ablated with 131I
• Post-thyroidectomy whole-body 131I scan detects residual thyroid tissue and distant metastases in differentiated thyroid cancer (papillary and follicular carcinoma)
• Lingual/ectopic thyroid: essential to confirm ectopic thyroid is the only functional thyroid tissue before any surgical intervention
• Poorly differentiated thyroid cancers lose iodine uptake - FDG-PET is preferred in such cases (sensitivity up to 84%, rising to 93% with CT fusion)

Summary Points for Exam

1. Thyroid scan = functional imaging, not primarily structural
2. Two main tracers: 99mTc (rapid, IV) and radioiodine (oral, 4-24 h)
3. Hot nodule: autonomous function, almost never malignant
4. Cold nodule: hypofunctioning, requires FNAC to exclude malignancy
5. Graves': diffuse hot; Toxic adenoma: focal hot; Thyroiditis: cold gland
6. 131I for post-thyroidectomy cancer surveillance and ablation
7. Subtraction scan: for parathyroid localisation in setting of thyroid disease
8. Modern practice: ultrasound + FNAC have largely replaced scan in nodule malignancy workup