Thyroid dysfunction, manifested by primary hypothyroidism, hyperthyroidism, goiter, or nodules, is a common delayed effect of radiation therapy fields that include the thyroid gland incidental to treating Hodgkin lymphoma (HL), brain tumors, head and neck sarcomas, and acute lymphoblastic leukemia.
Of children treated with radiation therapy, most develop hypothyroidism within the first 2 to 5 years posttreatment, but new cases can occur later. Reports of thyroid dysfunction differ depending on the dose of radiation, the length of follow-up, and the biochemical criteria utilized to make the diagnosis. The most frequently reported abnormalities include elevated thyroid-stimulating hormone (TSH), depressed thyroxine (T4), or both.[2,3,4,5] Compensated hypothyroidism includes an elevated TSH with a normal T4 and is asymptomatic. The natural history is unclear, but most endocrinologists support treatment. Uncompensated hypothyroidism includes both an elevated TSH and a depressed T4. Thyroid hormone replacement is beneficial for correction of the metabolic abnormality, and has clinical benefits for cardiovascular, gastrointestinal, and neurocognitive function.
The incidence of hypothyroidism should decrease with lower cumulative doses of radiation therapy employed in newer protocols. In a study of 1,677 children and adults with HL who were treated with radiation therapy between 1961 and 1989, the actuarial risk at 26 years posttreatment for overt or subclinical hypothyroidism was 47%, with a peak incidence at 2 to 3 years posttreatment. In a study of HL patients treated between 1962 and 1979, hypothyroidism occurred in 4 of 24 patients who received mantle doses less than 26 Gy but in 74 of 95 patients who received greater than 26 Gy. The peak incidence occurred at 3 to 5 years posttreatment, with a median of 4.6 years. A cohort of childhood HL survivors treated between 1970 and 1986 were evaluated for thyroid disease by use of a self-report questionnaire in the Childhood Cancer Survivor Study (CCSS). Among 1,791 survivors, 34% reported that they had been diagnosed with at least one thyroid abnormality. For hypothyroidism, there was a clear dose response, with a 20-year risk of 20% for those who received less than 35 Gy, 30% for those who received 35 Gy to 44.9 Gy, and 50% for those who received greater than 45 Gy to the thyroid gland. The relative risk (RR) for hypothyroidism was 17.1; for hyperthyroidism 8.0; and for thyroid nodules, 27.0. Elapsed time since diagnosis was a risk factor for both hypothyroidism and hyperthyroidism, where the risk increased in the first 3 to 5 years after diagnosis. For nodules, the risk increased beginning at 10 years after diagnosis. Females were at increased risk for hypothyroidism and thyroid nodules.