I-131 Thyroid Therapy Dose

Plan I-131 activity for benign and malignant thyroid disease using the Marinelli equation or fixed-activity protocols

Therapy parameters

Indication

Calculation method

Target dose (Gy)

Thyroid mass (g)

24-h RAIU (%)

Effective half-life (days)

Defaults: target 150 Gy for Graves; effective T½ ≈ 5 days when not measured. Adjust based on tracer study or clinic protocol.

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MBq

\( A_{0}\;[\text{MBq}] = \dfrac{D \times m \times 24.67}{T_{1/2,eff} \times \text{RAIU}\;[\%]} \)

D — target absorbed dose (Gy) · m — thyroid mass (g) · T_1/2,eff — effective half-life (days) · RAIU — uptake percentage

Indication	Target dose (Gy)	Notes
Graves disease	150–200	Aim for hypothyroidism (curative); lower doses → higher relapse rate
Toxic adenoma / nodule	300–400	Suppressed extranodular tissue spared
Toxic MNG	150–200	Often higher activity due to lower uptake
Benign non-toxic goitre	100–150	Volume reduction (~40–60 percent at 1 year)

Indication	MBq	mCi
Graves disease	370–740	10–20
Toxic adenoma / MNG	555–1110	15–30
Post-thyroidectomy ablation (low-risk DTC)	1110–1850	30–50
Ablation (intermediate-risk DTC)	1850–3700	50–100
DTC with locoregional / distant metastases	3700–7400	100–200

Stokkel MPM et al. EANM procedure guidelines for therapy of benign thyroid disease. Eur J Nucl Med Mol Imaging 2010;37:2218–2228. doi:10.1007/s00259-010-1536-8
Luster M et al. Guidelines for radioiodine therapy of differentiated thyroid cancer. Eur J Nucl Med Mol Imaging 2008;35:1941–1959. doi:10.1007/s00259-008-0883-1
SNMMI Procedure Standard for Therapy of Thyroid Disease with I-131 v2.0 (2012).
Marinelli LD, Quimby EH, Hine GJ. Dosage determination with radioactive isotopes. Am J Roentgenol 1948;59:260–280.

Calculator is for planning only. Pregnancy and breastfeeding are absolute contraindications. Verify activity with calibrator before administration.