SODIUM IODIDE I 131
Clinical safety rating: caution
Comprehensive clinical and safety monograph for SODIUM IODIDE I 131 (SODIUM IODIDE I 131).
Sodium iodide I 131 is a radioactive isotope that emits beta particles and gamma rays. It is taken up by the thyroid gland via the sodium-iodide symporter and incorporated into thyroid hormones. The beta radiation causes local destruction of thyroid tissue, reducing hormone production and treating hyperthyroidism or thyroid cancer.
| Metabolism | Sodium iodide I 131 is not metabolized; it is rapidly taken up by the thyroid and excreted primarily in urine. The remainder is excreted in feces and other body fluids. |
| Excretion | Primarily renal; approximately 90% excreted in urine within 72 hours, with the remainder eliminated via feces (biliary-fecal route, <10% in bile). |
| Half-life | Physical half-life: 8.02 days. Effective half-life in euthyroid patients: ~5-7 days, but reduced to ~3-5 days in hyperthyroidism due to increased turnover. In thyroid cancer with remnant ablation, effective half-life may be longer (up to 8 days) due to reduced clearance. |
| Protein binding | Minimal protein binding (<1%); primarily transported as free iodide in plasma. |
| Volume of Distribution | Vd approximately 0.2-0.4 L/kg, reflecting distribution into extracellular fluid and selective uptake by thyroid tissue (thyroid-to-plasma ratio ~25:1). |
| Bioavailability | Oral: near 100% absorption; >90% bioavailable as iodide. |
| Onset of Action | Oral: therapeutic effect (reduction in thyroid hormone levels) begins within 1-2 weeks, with maximal effect at 3-4 months. IV: N/A (not administered IV). |
| Duration of Action | Single dose effects persist for 4-6 months; definitive treatment for hyperthyroidism often achieved by 6 months. In thyroid cancer, therapeutic effects (ablation) are assessed at 3-6 months. |
For thyroid ablation or therapy of thyrotoxicosis: 100-200 mCi (3.7-7.4 GBq) orally as a single dose. For diagnostic imaging: 5-10 μCi (0.185-0.37 MBq) orally.
| Dosage form | CAPSULE |
| Renal impairment | No specific dose adjustment guidelines are available; however, because sodium iodide I 131 is primarily excreted renally, consider dose reduction in patients with severe renal impairment (eGFR < 30 mL/min/1.73 m²). Monitor urinary excretion and adjust accordingly. |
| Liver impairment | No adjustment required for hepatic impairment based on Child-Pugh classification. Sodium iodide I 131 is minimally metabolized by the liver. |
| Pediatric use | Dose is adjusted based on weight and condition: For treatment of hyperthyroidism, typical dose 0.15–0.3 mCi (5.55–11.1 MBq) per gram of estimated thyroid weight. For thyroid cancer, 50–150 mCi (1.85–5.55 GBq) based on body surface area or weight. Diagnostic dose: 0.05-0.1 μCi/kg (0.00185-0.0037 MBq/kg). |
| Geriatric use | Elderly patients may require lower doses due to decreased renal function. Monitor renal function and use the lowest effective dose. Caution in patients with underlying cardiac disease due to potential for radiation-induced thyroiditis and thyrotoxicosis exacerbation. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for SODIUM IODIDE I 131 (SODIUM IODIDE I 131).
| Breastfeeding | Radioiodine is excreted into breast milk; M/P ratio not established. Breastfeeding must be discontinued before administration and not resumed. The Nuclear Regulatory Commission recommends interruption of breastfeeding for the duration of the lactation period for therapeutic doses; diagnostic doses require cessation for at least 4 weeks. |
| Teratogenic Risk | FDA Pregnancy Category X. First trimester: I-131 crosses placenta; fetal thyroid begins concentrating iodide at 10-12 weeks; risk of fetal thyroid ablation, hypothyroidism, and cretinism. Second and third trimesters: substantial risk of fetal thyroid destruction with therapeutic doses; fetal radiation exposure may also increase risk of childhood cancer. Contraindicated in pregnancy. |
■ FDA Black Box Warning
Sodium iodide I 131 is a radioactive drug. It should be used only by physicians qualified in the safe use of radionuclides. Pregnancy is an absolute contraindication due to risk of fetal harm.
| Serious Effects |
["Pregnancy (absolute contraindication due to fetal thyroid ablation and teratogenicity)","Breastfeeding (must be discontinued permanently if therapy is given)","Known hypersensitivity to iodide or any component of the formulation","Inability to comply with radiation safety precautions"]
| Precautions | ["Radiation exposure to others: Patients must follow radiation safety precautions (e.g., avoid close contact with others, especially children and pregnant women, for a period after administration).","Bone marrow suppression: High doses may cause myelosuppression, requiring monitoring of blood counts.","Thyroid storm: Rare but possible in hyperthyroid patients; use caution in severe hyperthyroidism.","Secondary malignancies: Slightly increased risk of leukemia and other cancers with high cumulative doses.","Hypothyroidism: Expected after treatment; requires lifelong thyroid hormone replacement therapy."] |
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| Fetal Monitoring | Pre-administration: confirm negative pregnancy test in women of childbearing potential. During therapy: fetal monitoring not applicable as drug contraindicated in pregnancy. Post-therapy: assess fetal thyroid function if inadvertent exposure occurs (cord blood TSH, T4). |
| Fertility Effects | Possible transient or permanent ovarian failure with higher cumulative doses; adverse effects on spermatogenesis reported. The extent of gonadotoxicity depends on patient age and cumulative activity. Risk of genetic damage to germ cells, but human studies have not demonstrated significant hereditary effects. |