Thyroid Scintigraphy

Basics & Tracers - Glow & Go Juice

• Principle: Gamma camera detects $\gamma$-rays from tracers in thyroid, assessing function & morphology.
• Key Radiotracers:

⭐ $^{99m}$Tc pertechnetate is trapped by thyroid follicular cells via the NIS symporter but is not organified, unlike iodine isotopes.

Why & Why Not - Scan Signals

• Indications (Why - Interpreting Signals):
  • Thyrotoxicosis: Differentiate Graves'' disease (diffuse ↑ uptake) vs. Thyroiditis (↓ uptake).
  • Thyroid Nodules: Evaluate function (hot, warm, cold). Cold nodules warrant further investigation (e.g., FNAC).
  • Ectopic Thyroid Tissue: Detect uptake in aberrant locations (e.g., lingual, substernal).
  • Congenital Hypothyroidism: Assess for agenesis (no uptake) or dyshormonogenesis.
  • Post-Thyroidectomy: $I-131$ scan for remnant tissue or metastases (uptake indicates presence).
• Contraindications/Limitations (Why Not - Affecting Signals or Safety):
  • Pregnancy (especially $I-131$; $Tc-99m$ pertechnetate is relatively contraindicated).
  • Breastfeeding (requires temporary cessation; duration depends on radiopharmaceutical).
  • Recent Iodine Exposure: Contrast media, amiodarone → ↓ tracer uptake.
  • Thyroid Hormone Therapy: Suppresses TSH → ↓ tracer uptake.

⭐ Thyroid scintigraphy is crucial for differentiating causes of thyrotoxicosis, especially distinguishing Graves'' disease (high uptake) from thyroiditis (low uptake).

How-To & Normal View - Picture Perfect Thyroid

• Procedure:
  • Administer radiotracer (e.g., $^{\text{99m}}$Tc-pertechnetate, $^{\text{123}}$I, $^{\text{131}}$I).
  • Uptake time: 20 min for $^{\text{99m}}$Tc; 4-6h & 24h for $^{\text{123}}$I.
  • Imaging: Gamma camera, anterior view; patient supine, neck extended.
• Normal Findings:
  • Butterfly-shaped gland; two lobes, connecting isthmus.
  • Homogeneous, symmetric tracer distribution.
  • Normal uptake: $^{\text{123}}$I: 10-30% (24h); $^{\text{99m}}$Tc: 0.5-4% (20 min).
  • 📌 "Perfectly Pictured Thyroid": Uniform, symmetric.

⭐ A normal thyroid scan shows homogeneous, symmetric tracer uptake in both lobes, with the isthmus often visible.

Hot, Cold & Sick - Nodule Nuisance News

Interpreting thyroid scintigraphy focuses on nodule activity and diffuse changes.

• Nodule Types & Significance:

  Feature	Hot Nodule	Cold Nodule
  Tracer Uptake	↑ (Hyperfunctioning)	↓ (Hypofunctioning)
  Malignancy Risk	Low (<1%)	Higher (15-20%)
  TSH	Often ↓	Normal / ↑
  Common Cause	Toxic Adenoma, MNG	Cyst, Colloid, Cancer
  Next Step	Treat if toxic	US + FNA

  📌 Mnemonic: Cold is Cancer Concern.

• Diffuse Patterns:

  • Graves': Diffusely ↑ uptake.
  • Thyroiditis (Subacute): Initially ↓ uptake (thyrotoxic phase), then ↑ (recovery).
  • Hashimoto's: Patchy, heterogeneous uptake. May show cold areas.

⭐ Most thyroid cancers appear as 'cold' nodules on scintigraphy because malignant cells are typically less efficient at trapping or organifying iodine/technetium.

High‑Yield Points - ⚡ Biggest Takeaways

• ^99mTc-pertechnetate (trapping) is common; ¹²³I (trapping & organification) for cancer, dyshormonogenesis.
• Hot nodules are rarely malignant (<1%); cold nodules have higher malignancy risk (15-20%).
• Graves' disease shows diffusely increased, homogenous uptake in the entire thyroid gland.
• Toxic Multinodular Goiter (TMNG): multiple, discrete hot nodules with suppressed normal tissue.
• Subacute thyroiditis (de Quervain's) typically shows markedly decreased or absent tracer uptake.
• Discordant nodules (^99mTc hot, ¹²³I cold) suggest impaired organification, increasing malignancy concern.