[en] The effects of radiation on the thyroid were investigated in 102 patients treated by radiation therapy to the neck. All patients had radiation ports which included the thyroid gland. Serum thyroid stimulating hormone (TSH) levels were elevated in 41 cases and the cumulative elevation rate was 52.1% in 5 years. The high frequency of elevated serum TSH levels observed in patients whose thyroid glands were included within the radiation fields (74.1%) was statistically significant compared to those whose thyroid glands were only partially included (23.4%). Among the patients whose entire thyroid glands were included within the radiation field, combination with chemotherapy increased the frequency of elevated serum TSH levels, but the increase was not statistically significant. Among 36 laryngeal cancer patients treated by only radiation therapy through a portal encompassing part of the thyroid, 4 (14%) were found to have elevated serum TSH levels. We advocate routine monitoring of thyroid functions after radiation therapy to the neck. (author)