Can radiotherapy used as conditioning for bone marrow transplantation cause hypothyroidism?

Can Radiotherapy Cause Hypothyroidism in Bone Marrow Transplant Conditioning?

Yes, radiotherapy used as conditioning for bone marrow transplantation definitively causes hypothyroidism, with total body irradiation (TBI) conferring particularly high risk—up to 10-fold increased compared to no radiotherapy.

Magnitude of Risk

The risk of hypothyroidism following radiotherapy-based conditioning is substantial and well-documented:

• TBI alone increases hypothyroidism risk 5-10 fold compared to no radiotherapy 1
• Cranial radiotherapy (CRT) alone increases risk 2-3 fold 1
• Combined CRT + TBI increases risk more than 10-fold, representing the highest risk scenario 1
• Any spinal radiotherapy (SRT) exposure independently increases risk an additional 2-3 fold beyond other radiation modalities 1

Time Course and Prevalence

The development of hypothyroidism follows a predictable temporal pattern:

• Median onset occurs 2.7-2.9 years post-transplant for both compensated and uncompensated hypothyroidism 2
• Cumulative incidence reaches 8% at 3 years in myeloablative TBI-based regimens 3
• Long-term prevalence is 17.7% in pediatric survivors, with hypothyroidism risk increasing progressively with years after HSCT 4
• Thyroid dysfunction-free survival drops to 73.2% at 5 years and 59.2% at 10 years in pediatric ALL patients receiving allogeneic BMT 2

Independent Risk Factors

Beyond radiation exposure, several factors independently increase hypothyroidism risk:

• Male sex confers 3-fold increased risk [OR: 3.005,95% CI 1.145-7.890] 4
• Irradiation itself carries 2.9-fold increased risk [OR: 2.876,95% CI 1.120-7.386] 4
• Increasing time post-HSCT progressively elevates risk [OR: 1.148 per year, 95% CI 1.042-1.266] 4
• Age at transplant, BMT in second remission, and single-dose TBI (versus fractionated) are significantly associated with hypothyroidism onset 2

Radiation Dose and Fractionation Effects

The technical parameters of radiation delivery impact outcomes:

• Fractionated TBI (12-13.5 Gy in 6 fractions) causes less cataract formation than single-dose 10 Gy TBI, though hypothyroidism rates are comparable 5
• Both myeloablative and reduced-intensity TBI-based conditioning regimens carry similar hypothyroidism risks at 3 years (8% vs 5%, p=0.41), with relative risk of 1.6 for myeloablative versus reduced-intensity 3

Non-TBI Conditioning Regimens

Even chemotherapy-only conditioning carries thyroid risk:

• Busulphan and cyclophosphamide without TBI still resulted in hypothyroidism in 4 of 14 patients (29%) in one cohort 2
• This suggests chemotherapy-based conditioning has non-negligible thyroid toxicity, though lower than TBI-based regimens

Clinical Implications for Surveillance

Given these risks, systematic thyroid monitoring is essential:

• Biologic hypothyroidism occurs in 10% of adult TBI survivors, including primary hypothyroidism (6.5%), thyroiditis (3%), and Graves disease (0.5%) 5
• Most thyroid dysfunction is subclinical initially, without overt clinical symptoms 5
• Thyroid nodules develop in approximately 29% of patients (10 of 35 examined), with median detection at 7.8 years post-BMT 2
• Papillary thyroid cancer risk exists, with both documented cases occurring in TBI recipients 4

Common Pitfalls

• Assuming only TBI causes hypothyroidism: Cranial and spinal radiotherapy independently increase risk, and even chemotherapy-only regimens carry some thyroid toxicity 1, 2
• Overlooking male sex as a risk factor: Males have 3-fold higher risk than females, contrary to typical hypothyroidism epidemiology 4
• Inadequate long-term surveillance: Risk continues increasing beyond 5 years, requiring lifelong thyroid function monitoring 4, 2
• Missing subclinical disease: Most patients lack clinical symptoms despite biochemical dysfunction 5