¹³¹I (thyroid)

Indications

• Remnant ablation (destroy residual benign thyroid to simplify follow-up): not recommended routinely after total thyroidectomy for ATA low-risk DTC (strong recommendation, high certainty). If used, give a low activity.
• Adjuvant therapy (lower the risk of recurrence from suspected microscopic disease): may be considered for ATA low-intermediate and intermediate-high risk (conditional); recommended routinely for ATA high risk (strong).
• Treatment of known disease: recommended routinely after total thyroidectomy when distant metastases are present at diagnosis (strong).
• Oncocytic carcinoma: outcome data are limited. If RAI is not given empirically, a diagnostic scan may be used to assess iodine avidity (ATA 2025, conditional).
• Before deciding: measure postoperative Tg 6–12 weeks after total thyroidectomy (on LT4 or stimulated) and scan the neck with ultrasound. If Tg is above the excellent-response range or TgAb are present, image before giving RAI (ATA 2025).

Contraindications

• Pregnancy: absolute. Confirm a negative pregnancy test before treatment (ATA 2025).
• Breastfeeding or recent lactation: defer until breastfeeding or pumping has stopped for at least 3 months (ATA 2025).
• Recent iodine load: iodinated IV contrast within about 3 months or amiodarone can block uptake. Urinary iodine can confirm clearance (ATA 2025).
• Relative: significant renal failure (slower iodine clearance and higher marrow dose — consider dosimetry); comorbidity that makes hypothyroidism unsafe (prepare with rhTSH); a very large remnant (>15% uptake on the pre-therapy scan may favour more surgery); inability to follow radiation-safety instructions.

• rhTSH vs thyroid hormone withdrawal: rhTSH is preferred for remnant ablation and adjuvant therapy (strong, high certainty), and for patients whose comorbidity makes hypothyroidism risky. With known distant metastases, either may be used (conditional, low certainty). Target TSH >30 mIU/L (ATA 2025).
• rhTSH schedule (Thyrogen US label): 0.9 mg IM, repeated 24 h later; give ¹³¹I 24 h after the second injection.
• Withdrawal schedule (ATA 2025): stop LT4 for 3–4 weeks. If LT4 is stopped for ≥4 weeks, LT3 may cover the first weeks but must stop at least 2 weeks before RAI. Measure TSH before giving the activity.
• Low-iodine diet (ATA 2025): about 1–2 weeks at <50 µg iodine a day; extending beyond 2 weeks adds little. Do not restrict non-iodised salt: hyponatraemia has been reported, mainly in older patients on withdrawal, with metastatic disease, on thiazides or on the diet for more than a week.
• Empiric vs dosimetric activity (ATA 2025): empiric activities of 3.7, 5.6 or 7.4 GBq are common for metastatic disease. Dosimetry (maximum tolerated activity or lesional) should be considered in older patients, diffuse lung metastases and renal failure. In one series, 9.25 GBq (250 mCi) would have exceeded the maximum tolerated activity in 22% of patients under 70 and 50% of those aged 70 or over. A retrospective comparison found no 5-year survival gain from whole-body/blood dosimetry over a fixed 3.7 GBq.
• Benua–Leeper limits (maximum tolerated activity): blood absorbed dose, a surrogate for marrow, ≤2 Gy; whole-body retained activity ≤4.5 GBq, or ≤3.0 GBq (80 mCi) at 48 h when diffuse lung metastases are present, to avoid pneumonitis and fibrosis (ATA 2025; ATA 2015). The EANM Dosimetry Committee SOP describes blood dosimetry to the 2 Gy limit.
• Pre-therapy scan: ¹²³I or low-activity ¹³¹I may be considered (conditional). It changed management in 25–49% of patients in published series. An insignificant remnant with Tg <1 ng/mL may allow a lower activity or no RAI (ATA 2025).
• Post-therapy scan: recommended after every therapeutic activity (strong), generally 2–10 days after; add SPECT/CT when available (conditional). Post-therapy scans have shown uptake not seen on the pre-therapy scan in 13% of scans, and functioning metastases in 6.3% of patients with negative Tg (ATA 2025).

• Salivary glands: acute swelling, pain, dry mouth and taste change are common. In one cohort (median 5.21 GBq), 39% had acute salivary effects in the first year and <5% still had them at a median of 7 years. Symptoms occurred in 14% after 1.1 GBq and 40% after ≥2.8 GBq (ATA 2025).
• Salivary prevention: hydration is recommended. The role and timing of sour sweets are uncertain; one study found more gland damage when sour sweets were started within 1 h of RAI than at 24 h (ATA 2025). NSAIDs usually settle sialadenitis; a short steroid course or sialendoscopy is used for refractory cases.
• Dental: xerostomia raises the risk of caries. Involve a dentist (ATA 2025).
• Lacrimal: nasolacrimal duct obstruction presents with epiphora and predisposes to infection. Consider surgical correction (ATA 2025).
• Marrow: transient falls in white cells and platelets are usual; below 2 Gy to marrow the effect is minimal. Repeated treatments can leave persistent mild cytopenia. Check FBC and renal function before every therapy (ATA 2025).
• Gonads: temporary amenorrhoea in 8–27% of women in the first year; female fertility is not shown to be affected, but ovarian reserve falls slightly and menopause averages 49.5 vs 51 years. In men, FSH rises and sperm counts fall temporarily, usually normalising within 18 months. Counsel on infertility risk, and consider sperm banking, above a cumulative 14.8 GBq (400 mCi) (ATA 2025).
• Second primary cancer: a small, activity-related excess. Pooled relative risk 1.19 for any second cancer and 2.5 for leukaemia versus no RAI; excess absolute risk 14.4 solid cancers and 0.8 leukaemias per GBq per 10,000 person-years. Counsel patients; extra screening is not advised (ATA 2025).
• Lung: pneumonitis and fibrosis are rare, and linked to high activities with diffuse lung metastases (ATA 2015).
• Other: TSH stimulation can enlarge metastases; for brain metastases give SBRT and glucocorticoids before RAI (ATA 2025). Laxatives reduce bowel dose, especially after prolonged withdrawal; vigorous hydration reduces bladder and gonadal dose.

• Give oral and written radiation-safety instructions before preparation starts, following national rules (ATA 2025). Calculate precaution times from the activity, retention and home situation. Release home is preferred; release to a hotel is discouraged.
• Example: US NRC rules (10 CFR 35.75) allow release when the dose to any other person is unlikely to exceed 5 mSv. Written instructions are required if it may exceed 1 mSv, including breastfeeding advice when a nursing infant could be exposed.
• ¹³¹I emits β and γ radiation and contaminates body fluids, so control contamination as well as external dose (time, distance, shielding).
• Avoid pregnancy for at least 6 months after RAI (ATA 2025). In a Korean cohort, conception within 6 months was linked to more congenital malformations (odds ratio 1.74).
• Breastfeeding must stop before treatment, ideally 3 months before, to lower breast dose. A diagnostic ¹²³I scan can show residual breast uptake (ATA 2025).
• Men should wait at least 120 days before trying to conceive or giving sperm for assisted reproduction (ATA 2025).