What damage can radioactive iodine exposure cause to the thyroid gland and overall health?

Radioactive Iodine Exposure: Thyroid and Systemic Damage

Primary Thyroid Damage

Radioactive iodine exposure causes concentrated radiation damage to the thyroid gland, with the most critical consequence being thyroid cancer—particularly aggressive forms in children and adolescents that may be more severe than spontaneous thyroid cancers. 1

Mechanism of Thyroid Injury

• The thyroid gland concentrates radioactive iodine (particularly ¹³¹I and ¹²⁵I) with great efficiency through the Na+/I- symporter, which cannot distinguish between radioactive and stable iodine isotopes. 2
• This concentration mechanism results in localized radioactivity within the thyroid tissue, delivering high radiation doses directly to thyroid cells. 1, 3
• The thyroid absorbed dose can reach 1,500 cGy per 37 MBq with sufficient dietary iodine, but doubles to 2,900 cGy per 37 MBq in iodine-deficient populations. 4

Thyroid Cancer Risk

• Children, adolescents, and fetuses face the highest risk of malignant transformation, as the thyroid gland is particularly radiosensitive during these developmental periods. 1
• The fetal thyroid is more iodine-avid than the adult thyroid, resulting in higher radiation doses to the fetus during maternal exposure. 1
• Thyroid cancer incidence begins to increase several years after exposure, with papillary thyroid cancer being the predominant type. 2
• In adults, particularly those over 40 years of age, the risk of developing thyroid cancer is negligible. 5

Other Thyroid Effects

• Hypothyroidism can develop, especially in newborns and young children following exposure. 6
• Populations in iodine-deficient areas face higher risks because their thyroid glands take up more radioactive iodine than those with adequate iodine nutrition. 4, 5

Routes of Exposure

Near-Field vs. Far-Field Contamination

• Inhalation is the main route for those in the near field (close to the radiation source). 1
• Ingestion of contaminated food and drink—particularly milk—is the primary route for those in the far field (farther from the source). 1
• Contaminated air, food, and water all serve as sources of internal contamination with iodine-131. 2

Systemic Radiation Effects

Hematopoietic System Damage

• Radiation exposure causes bone marrow suppression leading to lymphopenia, neutropenia, and pancytopenia. 1
• Radiation-associated lymphopenia increases susceptibility to opportunistic infections. 1

Infectious Complications

• Prolonged immune suppression from radiation may lead to reactivation of cytomegalovirus (CMV), with the greatest risk occurring within the first 100 days post-exposure. 1
• Patients with absolute CD4 cell counts less than 0.200 × 10⁹ cells/L become highly susceptible to Pneumocystis carinii pneumonia. 1
• Recovery of CD4 cell counts may not occur for several months following exposure. 1

High-Risk Populations

Pregnant Women and Fetuses

• Pregnant women exposed to radioactive iodine face dual risks: direct maternal thyroid damage and enhanced fetal thyroid exposure due to the fetal thyroid's increased iodine avidity. 1
• Any pregnant woman exposed to more than 0.25 Gy of radiation requires fetal dose estimation by a health physicist. 1
• The most important factor for fetal survival is maternal survival; pregnant women should receive the same supportive care as non-pregnant adults. 1

Children and Adolescents

• This age group requires the most stringent protection measures due to their markedly higher sensitivity to radiation-induced thyroid cancer. 1, 7
• The risk of radiation-induced thyroid cancer justifies potassium iodide prophylaxis despite the risk of hypothyroidism, especially in newborns. 6

Iodine-Deficient Populations

• Individuals with inadequate dietary iodine intake face higher radioactive iodine uptake and consequently higher thyroid radiation doses. 4, 5
• Patients with untreated thyrotoxicosis, hypothyroidism, or endemic goiter also demonstrate higher radioactive iodine uptake than healthy individuals. 5

Critical Timing Considerations

Window for Protective Intervention

• Potassium iodide administered within 4 hours of radioactive iodine exposure reduces thyroid uptake by approximately 50%. 1, 8
• KI administered up to 48 hours before exposure can almost completely block thyroid uptake in iodine-sufficient adults. 4
• KI administration 16 hours or later after exposure has little to no protective effect. 4
• Administration 96 hours or more before exposure provides no significant protection. 4

Important Clinical Pitfalls

Inappropriate KI Use

• Potassium iodide is NOT a generic antiradiation drug and should only be used for confirmed radioiodine exposure. 1, 8
• In "dirty bomb" scenarios, radioiodines are extremely unlikely to be present due to their short 8.5-day half-life; KI administration in these cases risks life-threatening anaphylaxis without benefit. 1
• Individuals with iodine allergy history may experience severe allergic reactions including anaphylaxis. 1, 8

Age-Related Considerations

• For elderly patients, the risks of iodine-induced hyperthyroidism may outweigh the benefits of thyroid blocking. 6
• Newborns and infants are more sensitive to adverse effects from iodine administration. 5, 9