High-dose radiation exposure and hypothyroidism: aetiology, prevention and replacement therapy.

Without any doubt, high dose radiation exposure can induce hypothyroidism. However, there are open questions related to the mechanisms of its induction, corresponding dose thresholds and possible countermeasures. Therefore, this review addresses the aetiology, prevention and therapy of radiation induced hypothyroidism. External beam radiotherapy with several 10 Gy to the head and neck region and radioiodine therapy with several 100 Gy thyroid absorbed dose can destroy the thyroid gland and can induce autoantibodies against thyroid tissue. According to recent literature, clinical hypothyroidism is observed at threshold doses of ∼10 Gy after external beam radiotherapy and of ∼50 Gy after radioiodine therapy, children being more sensitive than adults. In children and adolescents exposed by the Chernobyl accident with mean thyroid absorbed doses of 500-800 mGy, subclinical hypothyroidism has been detected in 3%-6% of the cases with significant correlation to thyroid absorbed doses above 2.5 Gy. In case of nuclear emergencies, iodine thyroid blocking (ITB) is the method of choice to keep thyroid absorbed doses low. Large doses of stable iodine affect two different steps of internalization of radioiodine (transport and organification); perchlorate affecting the transport only may be an alternative to iodine. Administered before radioiodine incorporation, the effect of 100 mg iodide or more is still about 90% after 1 days, 80% after 2 days, and 50% or less after 3 days. If administered (too) late after exposure to radioiodine, the theoretically expected protective effect of ITB is about 50% after 6 h, 25% after 12 h, and about 6% after 24 h. In case of repeated or continuous exposure, repeated administration of 50 mg of iodide daily is indicated. If radiation-induced hypothyroidism cannot be avoided, thyroid hormone replacement therapy with individualized dosing and regular monitoring in order to maintain thyroid-stimulating hormone levels within the normal range ensures normal life expectancy.