Thyroid Function Disorders
Disorders of the thyroid are increasingly prevalent in childhood cancer survivors
Those especially at risk have had:
- Head and neck radiation therapy (RT)
- Total body irradiation
- Radioactive iodine treatment4
Reported abnormalities include:
- Hypothyroidism (under-activity of the thyroid)
- Hyperthyroidism (over-activity of the thyroid - much less common)
- Benign thyroid nodules
- Papillary thyroid cancer3
This is the most common complication involving the thyroid gland in cancer survivors.
Hypothyroidism may cause:
- Poor growth
- Lack of energy
- Poor school performance
- Weight gain
Low T4 and T3 levels can result in chronically elevated TSH levels - which in turn is associated with an increased risk of thyroid hyperplasia, benign nodules and thyroid carcinoma.
Generally due to direct damage to the gland from radiation therapy (RT)5.
Prevalent late effect of:
- Head and neck RT
- Mantle RT
Hodgkin Lymphoma (HL) survivors:
Patients treated for Hodgkin lymphoma (HL) are exposed to the highest levels of RT to this area, and at high risk of developing thyroid complications3
- This risk is dose-dependent:
- 30% of HL survivors developed hypothyroidism 20 years after diagnosis who received 35 to 44.99 Gy of RT
- 50% of HL survivors who received 45 Gy or more of radiation developed hypothyroidism6
- Females and older age at the time of therapy were also factors for an increased risk of hypothyroidism6
Pediatric brain tumor survivors (PBTS):
- Also more likely to develop hypothyroidism:
- Prevalence of 16% found in PBTS.8
- This also appears to be dose-dependent, with the risk of hypothyroidism doubling in those patients who received more than 25 Gy of craniospinal RT compared to those who received less than 25 Gy.8
Rhabdomyosarcoma (RMS) survivors:
- 9% of survivors of rhabdomyosarcoma (RMS) have hypothyroidism as a long term complication, compared to 1% of their siblings.9
- Over three quarters of these survivors had head and neck tumours
- Head and neck RT was the major risk factor for hypothyroidism in RMS survivors.9
Acute lymphoblastic leukemia (ALL) survivors:
Those survivors who had total body RT as part of their treatment are also at an increased risk of thyroid damage - hypothyroidism is the most common thyroid-related complication.7
- Most children treated for leukemia do not develop hypothyroidism or are at risk only for only mild central or primary hypothyroidism.
- Low dose cranial RT may be associated with both central and primary hypothyroidism
- Cranial RT damages pituitary
- A low peak response of TSH to TRH was found in 50% of children who had received 24 Gy CRT compared with 14% of children who had received 18 Gy
- Longer duration of follow-up (more than 5years) is associated with an increased incidence of this abnormality (30% versus 10 - 15%).
- Scattered RT to the thyroid from the cranial RT (in small children dose to the thyroid increases as the field edge approaches the thyroid and may reach 5% of the cranial dose.
- There is increased basal and peak serum TSH in response to TRH, compatible with subtle primary hypothyroidism in 25 – 50% of these patients.
- There would likely be better longitudinal growth if given children in this situation were given thyroid replacement therapy.
Hypothyroidism is such a common late effect in childhood cancer survivors and survivors warrant ongoing regular monitoring of thyroid function, particularly those at a particularly high risk mentioned above.
Occurs less frequently than hypothyroidism in childhood cancer survivors. However, one study showed that 5% of Hodgkin lymphoma survivors reported having hyperthyroidism, which is eight times greater than the sibling comparison group.6
Similar to hypothyroidism, exposure to higher doses of RT is associated with increased risk of hyperthyroidism.6