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Hypothyroidism is common late complication in head and neck patients treated with radiotherapy but no report about incidence rate in Thailand before. Aims of this retrospective descriptive study were to investigate the incidence rate of hypothyroidism and associated risk factors after radiotherapy for head and neck cancer. The retrospective study was conducted in head and neck cancers treated with radiotherapy and was tested serum level of thyroid function after treatment at Buddhachinaraj Phitsanulok Hospital from January 1, 2008 through December 31, 2016. Out of 151 head and neck cancers who had serum level of thyroid function after treatment, there were 123 men (81.46%) and 28 women (18.54%). Mean age of patient was 60 years. The most common was laryngeal cancer. The second and third most common was nasopharyngeal and hypopharyngeal cancer.102 patients (67.55%) were treated with chemoradiation, 24 patients (15.89%) were treated with surgery plus chemoradiation, 23 patients (15.23%) were treated with radiation alone. 49 of 151 patients had developed hypothyroidism (32.5%), median age was 59 years. Median time to develop hypothyroidism is 23 months (range 3-60 mo.) Most occured in 1 year post radiotherapy. Risk factor associate with hypothyroidism was sex. Incidence ocurred in male more than female (p value = 0.009). Age (p = 0.060),site of tumor (p = 0.216),treatment modality(p value= 0.520),surgery (p = 0.238), chemotherapy (p = 0.678) were not associate with hypothyroidism. In conclusion, incidence of hypothyroidism in this study is 32.5% or about 1/3 of all patients.Therefore, the physicians should be aware, regular follow-up and monitoring thyroid function pre and post irradiation to early diagnosis and treatment.
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