Hypothyroidism after radiation exposure: brief narrative review

Abstract

The thyroid gland is among the organs at the greatest risk of cancer from ionizing radiation. Epidemiological evidence from survivors of radiation therapy, atomic bombing, and the Chernobyl reactor accident, clearly shows that radiation exposure in childhood can cause thyroid cancer and benign thyroid nodules. Radiation exposure also may induce hypothyroidism and autoimmune reactions against the thyroid, but these effects are less well-documented. The literature includes only a few, methodologically weak animal studies regarding genetic/molecular mechanisms underlying hypothyroidism and thyroid autoimmunity after radiation exposure. Rather, evidence about radiation-induced hypothyroidism and thyroid autoimmunity derives mainly from follow-up studies in patients treated with external beam radiotherapy (EBRT) or iodine-131, and from epidemiological studies in the atomic bombing or nuclear accident survivors. Historically, hypothyroidism after external irradiation of the thyroid in adulthood was considered not to develop below a 10–20 Gy dose threshold. Newer data suggest a 10 Gy threshold after EBRT. By contrast, data from patients after iodine-131 “internal radiation therapy” of Graves´ disease indicate that hypothyroidism rarely occurs below thyroid doses of 50 Gy. Studies in children affected by the Chernobyl accident indicate that the dose threshold for hypothyroidism may be considerably lower, 3–5 Gy, aligning with observations in A-bomb survivors exposed as children. The reasons for these dose differences in radiosensitivity are not fully understood. Other important questions about the development of hypothyroidism after radiation exposure e.g., in utero, about the interaction between autoimmunity and hypofunction, and about the different effects of internal and external irradiation still must be answered.

The Iodine Rush: Over- or Under-Iodination Risk in the Prophylactic Use of Iodine for Thyroid Blocking in the Event of a Nuclear Disaster

“…), resulting in external exposure. Subsequent inhalation of contaminated air and ingestion of contaminated food and potable water can lead to internal radiation exposure and absorption of radioactive iodine, which primarily concerns the thyroid gland, being responsible of increased risk of thyroid alterations ( 70 ). Thus, there is no doubt that the thyroid gland is one of the most sensitive organs to, and at the highest risk of, cancer from ionizing radiation.…”

“…Even in later studies ( 70 ), for example in the population affected by radioiodine emissions after the Chernobyl NPP accident, it has been suggested that for any community with large radioiodine exposure, hypothyroidism in children is a likely event, and if found, can easily be treated. Therefore, it is useful to implement targeted screening tests for such populations.…”

Immune Checkpoint Inhibitors-Related Thyroid Dysfunction: Epidemiology, Clinical Presentation, Possible Pathogenesis, and Management

“…The thyroid gland is known to be more susceptible to autoimmune attacks than any other organ ( 62 ). Hypothyroidism often occurs after hemithyroidectomy, radioiodine therapy, and neck radiotherapy ( 15 , 63 ). Whether thyroid IRAEs have the same mechanism as AITDs, warrants further elucidation.…”

The Continuum of Thyroid Disorders Related to Immune Checkpoint Inhibitors: Still Many Pending Queries

“…Radiation can impair the thyroid gland via interfering with the mitosis of thyroid cells, disrupting the blood supply to the thyroid, and triggering autoimmunity characterized by the increased expression of anti-TPO Abs and anti-Tg Abs. Beyond the well-established elevated risk for external beam radiotherapy (EBRT)-induced hypothyroidism in cHL survivors more than 25 years after exposure to EBRT [ 133 ], radiotherapy in the form of TBI can cause permanent or transient hypothyroidism. A higher incidence of TBI-induced hypothyroidism has been related to a single dose of TBI (46–48%) compared to fractionated TBI (15–16%).…”