Radioiodine (I-123 / I-131): Thyroid¹²³I · ¹³¹I

Quick Answer

Iodine radioisotopes are trapped by the sodium-iodide symporter (NIS) and then organified into thyroid hormone, making them both diagnostic (uptake and scan) and therapeutic agents. I-123 (pure γ 159 keV, 13-h half-life, no beta) is preferred for diagnostic uptake and imaging; I-131 (β⁻ + γ 364 keV, 8-day half-life) is used for therapy — hyperthyroidism and differentiated thyroid-cancer remnant ablation/treatment — and for post-therapy whole-body scanning. Two board-defining concepts: trapping vs organification (why a rare nodule is pertechnetate-hot but iodine-cold), and the iodine-load trap (recent iodine suppresses uptake).

Mechanism

NIS actively co-transports iodide (with sodium) into the thyroid follicular cell — trapping — driven by the Na⁺/K⁺-ATPase gradient. Trapped iodide is then organified by thyroid peroxidase (TPO) onto tyrosine residues of thyroglobulin to form hormone. Pertechnetate (Tc-99m) is trapped by NIS but not organified, which is why a nodule can be pertechnetate-hot yet radioiodine-cold (discordant) — a discordant nodule carries malignancy concern and warrants further evaluation. I-131's beta emission (mean ~190 keV, mm-range) delivers the therapeutic dose to trapping tissue; its 364-keV gamma is imaged but degrades resolution and drives radiation-safety precautions.

Biodistribution

Thyroid (target), plus salivary glands, gastric mucosa (NIS-expressing), and urinary excretion; the nasopharynx and lactating breast also express NIS. Functioning differentiated thyroid-cancer metastases concentrate iodine — the basis of post-therapy whole-body scanning and I-131 therapy of iodine-avid disease. Physiologic bowel and bladder activity are common whole-body-scan mimics.

Physics

I-123's pure gamma and short half-life give lower thyroid dose and cleaner images; I-131's beta makes it therapeutic but its long half-life and high-energy gamma raise dose and require patient-release precautions.

Clinical Indications

• Thyroid uptake & scan (I-123): differentiate causes of hyperthyroidism (Graves vs toxic nodular vs thyroiditis) and assess nodule function (hot vs cold).
• I-131 therapy for hyperthyroidism: Graves disease, toxic nodular goiter.
• I-131 for differentiated thyroid cancer: remnant ablation, treatment of iodine-avid disease, and post-therapy whole-body scans.

Preparation & Protocol

• Screen for a recent iodine load — iodinated CT contrast, amiodarone, kelp/supplements — and defer if present (measure urinary iodine if unsure).
• Exclude pregnancy (absolute contraindication for therapy) and address lactation (NIS in breast; stop breastfeeding).
• For cancer studies/therapy, achieve TSH stimulation by thyroid-hormone withdrawal or recombinant human TSH (Thyrogen), often with a low-iodine diet.
• Uptake measurement at standardized times (e.g., 4–6 h and 24 h) against a reference standard.

Interpretation Highlights

• In thyrotoxicosis, the uptake value sorts the differential: high uptake (Graves, toxic nodular) is radioiodine-treatable; low uptake (thyroiditis, exogenous hormone, iodine load) is not — nothing is concentrating iodine.
• A discordant nodule (pertechnetate-hot, iodine-cold) is suspicious and should not be called benignly "hot."
• Functioning metastases on whole-body scan guide cancer therapy; correlate with thyroglobulin.

Reporting Checklist

• Report percent uptake with the timing and reference range.
• State the iodine-load / medication history and pregnancy status.
• For therapy, document written directive, activity, TSH-stimulation method, and radiation-safety/patient-release measures (I-131 at higher activities).

Common Pitfalls

• Iodine load (CT contrast, amiodarone, kelp/supplements) suppressing uptake — can invalidate a study or reduce therapy efficacy.
• Inadequate TSH stimulation (insufficient withdrawal or rhTSH) reducing cancer-therapy uptake.
• Pregnancy/lactation not excluded before administration.
• Misreading physiologic salivary/gastric/bowel/bladder activity on whole-body scans as disease.

Board Pearls

Iodide is trapped by the sodium-iodide symporter (NIS) and then organified by thyroid peroxidase. Pertechnetate is trapped but not organified — so a rare nodule is pertechnetate-hot yet radioiodine-cold (discordant), a finding that raises malignancy concern. Use I-123 for diagnosis (pure gamma, best dosimetry/images) and I-131 for therapy (its beta delivers the dose).

In thyrotoxicosis the uptake sorts the differential: high uptake (Graves, toxic nodular) is radioiodine-treatable; low uptake (thyroiditis, exogenous hormone, iodine load) is not — there is nothing concentrating iodine. Always screen for a recent iodine load (contrast, amiodarone, kelp) that competes for NIS and suppresses uptake.

For differentiated thyroid cancer, TSH stimulation (withdrawal or rhTSH) plus a low-iodine diet maximizes uptake; functioning metastases on the post-therapy whole-body scan are the basis for I-131 therapy of iodine-avid disease. Pregnancy is an absolute contraindication to therapy; I-131's long half-life and 364-keV gamma drive patient-release rules (≤ ~5 mSv to others / activity or dose-rate thresholds).

Related Pages

• Disease: Hyperthyroidism, Thyroid nodule, Thyroiditis.
• Related: pertechnetate thyroid imaging; radiation safety & patient release.

Figure / Diagram Suggestions

• A NIS trapping → TPO organification cell schematic (with the pertechnetate "trapped-not-organified" branch).
• A high-vs-low uptake thyrotoxicosis decision tree.
• An I-123 vs I-131 property/role comparison table.

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