incidence of hypothyroidism after external radiation to thyroid region

background: in this study we evaluated the incidence of hypothyroidism in patients who received external beam radiation due to cancer, when radiation portals included thyroid gland. methods: this prospective study was conducted from april 2004 to december 2005 among patients with head and neck or breast malignancies who referred to radiation oncology department of cancer institute, imam khomeini hospital, treated with external beam radiotherapy. all patients received radiation to the thyroid gland. thyroid function was tested at the beginning of treatment, 3 months and 6 months after the completion of radiotherapy. results: in all 34 patients were included in the study. the median age of patients was 53 years. eighteen patients were female and 16 were male. all patients received external radiotherapy. they received radiation ranging from 5000 cgy (14 patients 41.2%) to 7400 cgy (1 patient 2.9%) with median of 6000 cgy. physical examination of thyroid at the beginning of treatment was normal and all of the patients were euthyroid . the results from tsh test showed significant difference at baseline, after 3 and 6 months follow-up (p = 0.001) conclusion: the findings indicated that none of the patients developed hypothyroidism in a period of six months after completion of radiotherapy. however, it seems that 6 months fallow-up is early for judgment about evaluation of patients for hypothyroidism state and it needs further follow up for minimum of 12 months.