Can radiation therapy cause hyperthyroidism or hypothyroidism?

Radiation Therapy Can Cause Both Hypothyroidism and Hyperthyroidism

Radiation therapy can cause both hypothyroidism (more commonly) and hyperthyroidism (less commonly), with hypothyroidism being the predominant thyroid dysfunction occurring in up to 60% of patients receiving neck irradiation. 1

Hypothyroidism After Radiation

Incidence and Risk Factors

• Hypothyroidism is the most common radiation-induced thyroid dysfunction, affecting 20-30% of patients receiving curative radiotherapy to the neck region 2
• Approximately half of hypothyroidism cases develop within the first 5 years after therapy, but can occur up to 25 years later 2
• Risk increases with:
  • Higher radiation dose (clinical hypothyroidism observed at threshold doses of ~10 Gy) 3
  • Smaller thyroid gland volume 4
  • Younger age at exposure (children more sensitive than adults) 3
  • Radiation field that includes the thyroid gland 1

Mechanism

• Radiation causes vascular damage, direct parenchymal cell destruction, and autoimmune reactions against thyroid tissue 2
• The risk appears dose-dependent, with a significant dose-volume effect 1
• In a study evaluating the relationship between radiation doses and hypothyroidism risk, patients had an 11.5% risk if the percentage of thyroid receiving 30 Gy was ≤62.5%, versus 70.8% risk if >62.5% 1

Hyperthyroidism After Radiation

Incidence and Risk Factors

• Less common than hypothyroidism, occurring in approximately 1-1.5% of patients 1
• Local radiation doses of 40 Gy have been associated with hyperthyroidism 1
• Can manifest as Graves' disease or thyroiditis 2, 5

Mechanism

• Radiation can induce autoantibodies against thyroid tissue 3
• May trigger autoimmune reactions leading to Graves' disease 5
• Hyperthyroidism can induce bone loss by activation of osteoclast activity 1

Monitoring Recommendations

Who Should Be Monitored

• All patients who receive radiation to the neck region or where the radiation field includes the thyroid gland 1
• Patients who received cranial, orbital, infratemporal, or nasopharyngeal irradiation (risk of hypothalamic-pituitary injury) 1

Monitoring Protocol

• Baseline thyroid function assessment before treatment 2
• TSH and thyroid/neck exam yearly after radiation therapy 1
• Monitor for both hypothyroidism and hyperthyroidism signs and symptoms
• Long-term monitoring is essential as thyroid dysfunction can develop decades after exposure 5

Clinical Implications

Impact on Health

• Untreated hypothyroidism can lead to fatigue, weight gain, mental slowing, heart failure, and elevated lipid levels 1
• Untreated hyperthyroidism can lead to atrial fibrillation, congestive heart failure, osteoporosis, and neuropsychiatric problems 1
• Radiation-induced thyroid dysfunction has been linked to increased risk of cardiac disease, mortality, and decreased quality of life 4

Management

• For hypothyroidism: Hormone replacement therapy with individualized dosing and regular monitoring to maintain TSH levels within normal range 3
• For hyperthyroidism: Treatment similar to spontaneously occurring hyperthyroidism 2

Prevention Strategies

• Modern radiotherapy techniques (stereotactic radiosurgery, three-dimensional conformal irradiation, intensity modulated radiotherapy) allow better dose distribution with lower dose to non-target organs 2
• Consider thyroid dose constraints during treatment planning to keep risk of radiation-induced hypothyroidism below 25% 4
• Precise delineation of the thyroid gland during treatment planning is crucial for accurate risk assessment 4

Radiation-induced thyroid disorders remain underestimated and underreported despite their significant functional consequences 2. Clinicians should maintain a high index of suspicion and implement appropriate monitoring protocols for all patients receiving radiation therapy that may affect the thyroid gland.