[en] Radiation therapy in combination with surgery has an important role in the therapy of the head and neck cancer. We conducted a prospective study for patients with head and neck cancer treated with surgery and radiation to evaluate the effect of therapies on the thyroid gland, and to identify the factors that might influence the development of hypothyroidism. From September 1986 through December 1994, 71 patients with head and cancer treated with surgery and radiation were included in this prospective study. Patients' age ranged from 32 to 73 years with a median age of 58 years. There were 12 women and 59 men. Total laryngectomy with neck dissection was carried out in 45 patients and neck dissection alone in 26 patients. All patients were serially monitored for thyroid function before and after radiation therapy. Radiation dose to the thyroid gland ranged from 40.6Gy to 60Gy with a median dose of 50Gy. The follow-up duration was 3 to 80 months. The overall incidence of hypothyroidism was 56.3% (40/71); 7 out of 71 patients (9.9%) developed clinical hypothyroidism and 33 patients (46.4%) developed subclinical hypothyroidism. No thyroid nodules, thyroid cancers, or hyperthyroidism was detected. The risk factor that significantly influenced the incidence of hypothyroidism was a combination of surgery (total laryngectomy with neck dissection) and radiation therapy (P=0.0000). Four of 26 patients (15.4%) with neck dissection alone developed hypothyroidism while 36 of 45 patients (80%) with laryngectomy and neck dissection developed hypothyroidism. The hypothyroidism following surgery and radiation therapy was a relatively common complication. The factor that significantly influenced the incidence of hypothyroidism was combination of surgery and radiation therapy. Evaluation of thyroid function before and after radiation therapy with periodic thyroid function tests is recommended for an early detection of hypothyroidism and thyroid hormone replacement therapy is recommended whenever hypothyroidism develops. (author)