Drug reference
radioactive iodine (131i)
antithyroid drug, radiopharmaceutical · antithyroid, thyroid cancer therapy
antithyroid drug, radiopharmaceuticalantithyroid, thyroid cancer therapy
CDSCO approved
EXCRETION
—
not curated
INTERACTIONS
—
none in our sources
PREGNANCY
—
not curated
Mechanism
Emits both γ rays and β particles. Rapidly and efficiently trapped by the thyroid, incorporated into iodoamino acids, and deposited in the colloid of the follicles. Destructive β particles originate within the follicle and act almost exclusively on the parenchymal cells of the thyroid, causing damage to destroy the gland.
Indications
hyperthyroidism (especially in older patients, those with heart disease, or Graves disease persistent/recurrent after subtotal thyroidectomy or prolonged antithyroid drug treatment, toxic nodular goiter)thyroid destruction for an overactive or enlarged thyroidthyroid ablation and treatment of metastatic thyroid cancer
Dosing
- Adult
- Hyperthyroidism: 4 to 15 mCi (recommended target 8 mCi to the thyroid gland). Thyroid cancer ablation: 30 to 150 mCi or more.
Pharmacokinetics
Half-life
8 days
Contraindications
- pregnancy (after the first trimester, risk of fetal thyroid damage; even during the first trimester, adverse effects of radiation on fetal tissues)
- treatment of children (controversial due to theoretical concern about neoplastic changes)
Side effects
Common
delayed hypothyroidism (high incidence)
Serious
- small but significant increase in specific types of cancer (stomach, kidney, breast)
- radiation thyroiditis (with release of preformed T4 and T3, worsening of hyperthyroidism symptoms, cardiac manifestations, rarely thyroid storm)
Related guidelines
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Sources: Goodman & Gilman 14e·Verified: 2026-05-10 · House clinical team