What are the complications of radioactive iodine (Iodine-131) therapy?

Complications of Radioactive Iodine Therapy

Radioactive iodine (I-131) therapy causes both early and late complications affecting multiple organ systems, with risks increasing proportionally to administered dose and cumulative activity, though it remains generally safe and effective when appropriately selected and dosed.

Early Complications (Within Days to Weeks)

Gastrointestinal Effects

• Nausea and vomiting are the most common early side effects, occurring in approximately 35% of treatments and correlating directly with the administered radioactivity dose 1
• Radiation sickness may occur with higher doses 2

Thyroid and Neck Effects

• Radiation thyroiditis develops in approximately 19% of treatments, presenting as neck pain and swelling 1
• Acute thyroid crisis represents a rare but potentially life-threatening complication 2

Salivary Gland Toxicity

• Sialadenitis (salivary gland inflammation) occurs in approximately 24% of treatments 1
• Dry mouth affects approximately 14% of treatments 1
• These symptoms correlate with the amount of radioactivity administered in each treatment 1

Hematologic Effects

• Transient bone marrow suppression occurs in approximately 9% of treatments, manifesting as leukopenia, thrombocytopenia, and anemia 2, 1
• Blood dyscrasias including chromosomal abnormalities may develop 2

Late Complications (Months to Years)

Permanent Salivary Dysfunction

• Permanent salivary gland dysfunction develops in approximately 2% of pediatric patients and correlates with the number of treatments and cumulative I-131 activity 1
• This complication does not correlate with individual treatment doses but rather with repeated exposures 1

Hematologic Malignancies

• Permanent bone marrow suppression occurs in approximately 4% of pediatric patients receiving multiple treatments 1
• Acute leukemia represents a potential late complication 2
• These risks correlate with cumulative radioiodine doses, particularly exceeding 37.0 GBq (1,000 mCi) 3

Pulmonary Complications

• Pulmonary fibrosis develops in approximately 5% of pediatric patients, correlating with the number of treatments and cumulative I-131 activity 1
• This complication is particularly concerning in patients with diffuse pulmonary metastases 4

Secondary Primary Malignancies

• Second cancers occur in approximately 4% of pediatric patients 1
• The risk of secondary primary malignancy (SPM) remains controversial, with some studies showing no increased incidence while others suggest increased risk with cumulative doses exceeding 5.6-7.4 GBq (150-200 mCi) 3
• No specific secondary cancer type has been definitively linked to RAI exposure, though caution is warranted with cumulative doses exceeding 37.0 GBq (1,000 mCi) 3

Reproductive Effects

• Fertility alterations occur in approximately 5% of pediatric patients, though these do not correlate with cumulative activity or number of treatments 1
• Gonadal dysfunction represents a recognized risk 4

Ocular and Nasolacrimal Effects

• Eye and nasolacrimal system complications are recognized risks, though specific incidence rates vary 4

Thyroid Function Outcomes

Post-Treatment Thyroid Status

• Hypothyroidism develops in 25.5% of patients within six months of treatment, with higher frequency (83%) in those receiving doses ≤20 mCi 5
• Euthyroid state is achieved in 19.6% of patients by six months, more commonly (64.5%) with doses >20 mCi 5
• Persistent hyperthyroidism occurs in 9.2% of patients at six months 5

High-Risk Populations

Pregnant Women

• Radioiodine is absolutely contraindicated during pregnancy as it readily crosses the placenta and concentrates in the fetal thyroid gland 6
• Breastfeeding must be discontinued during and following dose administration 2
• The 8-day half-life of I-131 poses prolonged exposure risk to nursing infants 6

Children and Adolescents

• Pediatric patients face higher risk of aggressive thyroid cancer from radioiodine exposure compared to adults 7
• I-131 is not usually used for hyperthyroidism treatment in patients under 30 years of age 2
• All complication rates (except fertility problems) correlate with cumulative activity and number of treatments in pediatric populations 1

Contamination Hazards

Environmental Contamination

• Significant contamination occurs via perspiration, saliva, breath, and urine during the first 24-48 hours post-therapy 8
• Removable activity from skin reaches maximum at 24 hours post-therapy and correlates positively with treatment dose 8
• Room surfaces may exceed contamination levels requiring cleanup in restricted areas 8

Healthcare Worker Protection

• Strict isolation precautions are mandatory, including gowns, masks, caps, double gloves, and shoe covers 6
• Thyroid bioassays on medical staff show no significant uptake when proper precautions are followed 8

Critical Clinical Pitfalls

The most important pitfall is administering repeated high-dose treatments without considering cumulative toxicity—late complications correlate strongly with total cumulative activity rather than individual treatment doses, necessitating careful tracking of lifetime I-131 exposure 1, 3.

Death, though rare, represents a potential outcome of severe complications including radiation sickness, bone marrow depression, and acute thyroid crisis 2.