IODOTOPE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for IODOTOPE (IODOTOPE).
Iodine-131 is taken up by the thyroid gland and emits beta particles and gamma rays, causing destruction of thyroid tissue via radiation-induced cell death.
| Metabolism | Iodine-131 is not metabolized; it is excreted primarily in urine as iodide. Minimal fecal excretion. Half-life: approximately 8 days (physical) but biological half-life in thyroid is longer (up to 100 days). |
| Excretion | Primarily renal: >90% excreted in urine as iodide. Fecal excretion is negligible (<2%). |
| Half-life | Terminal half-life is approximately 120-140 days for total body iodine, but the effective half-life for therapeutic use is 8-13 days due to biological turnover in the thyroid. For diagnostic use, effective half-life is 1-2 days. |
| Protein binding | Minimal protein binding (<5%); primarily transported as free iodide. Major binding proteins are thyroxine-binding globulin (TBG) and albumin if converted to thyroid hormones, but for iodide itself, binding is negligible. |
| Volume of Distribution | Vd is approximately 0.2-0.4 L/kg, concentrated in thyroid, salivary glands, stomach, and kidneys. Clinically, high thyroid uptake indicates normal function. |
| Bioavailability | Oral: >90% absorbed. IV: 100%. |
| Onset of Action | Oral: Therapeutic effect (thyroid ablation) begins within 2-4 weeks. Diagnostic: thyroid uptake can be measured starting at 2-6 hours post-dose. |
| Duration of Action | Therapeutic effect of radioactive iodine (I-131) lasts indefinitely due to thyroid tissue destruction. Diagnostic effect of I-123 or I-131 for imaging: activity clears by 24-48 hours, but thyroid uptake persists for several weeks. |
For thyroid ablation: 3.7-5.55 MBq (100-150 μCi) orally as a single dose. For hyperthyroidism: 185-555 MBq (5-15 mCi) orally as a single dose.
| Dosage form | CAPSULE |
| Renal impairment | Based on GFR: GFR >60 mL/min: no adjustment; GFR 30-60 mL/min: reduce dose by 25%; GFR <30 mL/min: reduce dose by 50%. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 25%; Child-Pugh C: reduce dose by 50%. |
| Pediatric use | For hyperthyroidism: 0.15-0.3 MBq/kg (4-8 μCi/kg) orally as a single dose. For thyroid cancer: 1.11-3.7 MBq/kg (30-100 μCi/kg) orally as a single dose. |
| Geriatric use | Start at lower end of dosing range (e.g., 3.7 MBq for ablation) due to decreased renal function; monitor for radiation effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for IODOTOPE (IODOTOPE).
| Breastfeeding | Radioactive iodine is excreted in breast milk. Breastfeeding must be permanently discontinued after therapeutic doses (≥30 mCi) due to prolonged excretion and radiation exposure to infant. For diagnostic doses (≤5 mCi), temporary cessation of breastfeeding for 2-4 weeks may be recommended based on half-life and dose. M/P ratio not clinically established due to radioactivity concerns. |
| Teratogenic Risk | Iodine-131 (Iodotope) crosses the placenta. First trimester: Risk of fetal thyroid ablation if >5 mCi administered; fetal thyroid begins concentrating iodine at 10-12 weeks. Second/third trimesters: High risk of fetal hypothyroidism and potential neurodevelopmental deficits due to thyroid gland concentrating radioiodine. Avoid use in pregnancy unless life-threatening maternal condition warrants exposure with fetal risk considered. |
■ FDA Black Box Warning
WARNING: RADIOACTIVE IODINE MAY CAUSE PERMANENT HYPOTHYROIDISM. PATIENTS SHOULD BE COUNSELED ON RADIATION SAFETY PRECAUTIONS TO MINIMIZE EXPOSURE TO OTHERS. USE IN PREGNANCY IS CONTRAINDICATED DUE TO RISK OF FETAL HYPOTHYROIDISM AND RADIATION INJURY.
| Serious Effects |
Pregnancy (absolute). Lactation (absolute). Known hypersensitivity to iodine. Severe renal impairment (relative).
| Precautions | May cause permanent hypothyroidism. Risk of radiation-induced thyroiditis. Bone marrow suppression at high doses. Contraindicated in pregnancy and lactation. Avoid in patients with severe vomiting or diarrhea due to risk of radiation exposure. Ensure adequate hydration to minimize bladder radiation. |
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| Fetal Monitoring | Monitor maternal thyroid function (TSH, T4) before and after therapy. In inadvertent fetal exposure, assess fetal thyroid function via cord blood sampling or neonatal screening. Fetal ultrasound for goiter detection. Monitor maternal complete blood count for bone marrow suppression. Radiation safety precautions for maternal contact with others. |
| Fertility Effects | High cumulative doses (>200 mCi) may cause transient or permanent ovarian failure in women, and testicular dysfunction in men including oligospermia or azoospermia. Risk of infertility increases with dose and age at treatment. Pre-treatment fertility preservation options should be discussed. Gonadal exposure may be minimized with hydration and frequent voiding. |