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Endocrine · Thyroid

Thyroid Uptake & Scan

Snapshot

Thyroid scintigraphy and radioiodine uptake assess how the gland traps and organifies iodine. They establish the cause of hyperthyroidism (Graves’ disease vs thyroiditis vs toxic nodule), characterise nodules as “hot” (autonomous, almost always benign) or “cold”, and evaluate retrosternal/ectopic thyroid tissue.

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⁹⁹ᵐTc-pertechnetate is trapped (not organified) and gives a quick trapping image; ¹²³I is trapped and organified, better reflecting hormone synthesis and used for uptake quantification. Uptake is high in Graves’, low in thyroiditis or exogenous thyroxine.

Cause of thyrotoxicosisHigh vs low uptake
Hot noduleAlmost always benign
Cold noduleUS risk ± FNA

When to image

  • Determining the cause of thyrotoxicosis (Graves vs thyroiditis vs toxic nodular disease).
  • Characterising nodule function, especially with low/low-normal TSH or multinodular goitre (to identify autonomous “hot” nodules and avoid unnecessary FNA).
  • Assessing retrosternal goitre or ectopic thyroid.

How to read it

  • Diffuse high uptake → Graves’ disease.
  • Low/absent uptake → thyroiditis, exogenous thyroxine, or recent iodine load.
  • Focal high uptake with suppressed background → toxic autonomous nodule.
  • A non-functioning (“cold”) nodule is not specific for cancer — manage by ultrasound (TI-RADS) risk and FNA where indicated.

Protocol

  • ⁹⁹ᵐTc-pertechnetate: image 15–20 min after IV injection. Oral ¹²³I: image at 2–6 h and/or 24 h; uptake (RAIU) usually measured at 4–6 h and 24 h.
  • Avoid iodine load: IV iodinated contrast within 1–2 months, amiodarone within 3–6 months; review other interfering medications.
  • Correlate with TSH, free hormones and antibodies.

Pitfalls

  • Recent iodine (contrast, amiodarone) suppresses uptake.
  • Rare discordant nodules (pertechnetate-hot but iodine-cold).
  • Interpret with biochemistry and ultrasound.
Evidence & guidelines
  • EANM/SNMMI practice guideline for RAIU and thyroid scintigraphy (Giovanella et al., Eur J Nucl Med Mol Imaging 2019;46:2514–2525).
  • Scintigraphy is selective — most nodules are assessed by ultrasound and FNA.
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