Radioactive Iodine (I-131)
Key Points
- Radioactive iodine (RAI) is a targeted radiopharmaceutical that concentrates in thyroid tissue
- Primary use in pediatrics is for definitive treatment of Graves' disease
- Treatment decisions require careful risk-benefit assessment due to radiation exposure
- Long-term follow-up is essential due to risk of hypothyroidism and other complications
Mechanism of Action
Primary Effects
- I-131 emits beta radiation that causes DNA damage and cell death
- Selectively concentrates in thyroid follicular cells via NIS transporter
- Tissue penetration: 0.5-2.0 mm
- Effective half-life in thyroid: 6-8 days
Cellular Effects
- Direct radiation-induced DNA damage
- Oxidative stress and free radical generation
- Apoptosis of thyroid follicular cells
- Progressive fibrosis of thyroid tissue
Clinical Indications
Primary Indications
- Graves' disease refractory to medical therapy
- Thyroid cancer (papillary and follicular)
- Toxic nodular goiter (rare in children)
Specific Scenarios
- Failed antithyroid drug therapy
- Poor medication compliance
- Significant adverse reactions to antithyroid drugs
- Patient/family preference for definitive therapy
Administration & Dosing
Pre-treatment Evaluation
- Pregnancy test in females of childbearing age
- Recent thyroid function tests
- Thyroid uptake scan
- Documentation of informed consent
Dosing Guidelines
- Graves' Disease:
- 80-200 µCi/g of thyroid tissue
- Typical total dose: 10-15 mCi
- Thyroid Cancer:
- Initial ablation: 30-100 mCi
- Metastatic disease: 150-200 mCi
Administration Protocol
- Hold antithyroid drugs 3-5 days before treatment
- NPO for 2 hours before and after administration
- Oral administration as capsule or liquid
- Radiation safety precautions for 5-7 days
Monitoring & Follow-up
Initial Follow-up
- Weekly TFTs for first month
- Monitor for radiation thyroiditis
- Assessment of symptoms
- Begin levothyroxine when indicated
Long-term Monitoring
- TFTs every 3-6 months
- Annual physical examination
- Screening for secondary complications
- Growth and development monitoring
Contraindications & Precautions
Absolute Contraindications
- Pregnancy
- Breastfeeding
- Inability to comply with radiation precautions
Relative Contraindications
- Age < 5 years
- Large goiter with compressive symptoms
- Active thyroid eye disease
- Inability to maintain isolation
Complications & Side Effects
Short-term Complications
- Radiation thyroiditis (5-10%)
- Sialadenitis
- Nausea and vomiting
- Thyroid storm (rare)
Long-term Complications
- Hypothyroidism (>90% at 1 year)
- Secondary malignancies (very rare)
- Fertility concerns
- Chronic dry eyes/mouth
Further Reading
