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Thyroid Nuclear Scan: Procedure, Uses, and What to Expect

Purpose

Evaluates thyroid structure and function using radioactive tracers. Complements ultrasound (anatomy) and blood tests (hormones).

Indications

  • Hyperthyroidism etiology (Graves', toxic nodule, thyroiditis).
  • Thyroid nodule characterization (hot vs. cold).
  • Thyroid cancer staging/metastasis survey (post-thyroidectomy, I-131 whole-body scan).
  • Ectopic thyroid tissue (lingual, substernal).
  • Congenital hypothyroidism (neonatal scan).

Procedure

  1. Tracer: Usually I-123 (gamma, 13 hr half-life) or Tc-99m pertechnetate (6 hr). I-131 used for therapy/post-therapy scans.
  2. Administration: Oral capsule (I-123) or IV injection (Tc-99m).
  3. Uptake period: 2–4 hours (I-123) or 20 min (Tc-99m).
  4. Imaging: Gamma camera; anterior, oblique views; 10–30 min.
  5. Quantitative uptake: Probe measurement at 2h and 24h (I-123) for % uptake.

Preparation

  • Stop thyroid meds (levothyroxine 4–6 weeks; antithyroid drugs 5–7 days) per protocol.
  • Low-iodine diet 1–2 weeks (for I-123/I-131).
  • No recent iodinated contrast (CT).
  • Pregnancy/breastfeeding: contraindicated (except urgent cancer staging).

Safety

Low radiation dose (I-123: ~0.5–1 mSv). Side effects rare: nausea, neck discomfort, allergic reaction to tracer. No isolation needed after I-123/Tc-99m.

Interpretation

  • Hot nodule: Autonomous function (toxic adenoma) – rarely malignant.
  • Cold nodule: Non-functioning – ~5–10% malignant; needs FNA.
  • Diffuse increased uptake: Graves' disease.
  • Decreased/absent uptake: Thyroiditis, exogenous hormone, factitious thyrotoxicosis.

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