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Purpose: To evaluate the incidence of thyroid disorders and dose distribution to the thyroid in patients treated with radiotherapy for head-and-neck carcinomas. Methods and Materials: A retrospective evaluation of data from 73 patients treated for head-and-neck cancers in our department was performed. Thyroid function was evaluated mainly by the measurement of thyrotropin (thyroid stimulating hormone [TSH]). A retrospective analysis of treatment plans was performed for 57 patients. Percentages of thyroid glandular volume absorbing 10, 30, and 50 Gy (V10, V30, and V50 respectively) were considered for statistical analysis. Results: A majority of patients (61%) had a normal thyroid function whereas 19 patients (26%) had hypothyroidism. Mean thyroid volume was 30.39 cc. Point 3 (located at isthmus) absorbed lower doses compared with other points (p< 0.0001). Median values of V10, V30, and V50 were 92% (range, 57–100%), 75% (range, 28.5–100%), and 35% (range, 3– 83%) respectively. Gender was associated with toxicity (presence of any kind of thyroid disorders) (p < 0.05), with females displaying higher levels of TSHr (relative TSH patient’s value/maximum value of the laboratory range) (p 0.0005) and smaller thyroid volume (p 0.0012) compared with male population. TSHr values were associated with thyroid volume, and the presence of midline shielding block in the anterior field was associated with relative free thyroxine (FT4r patient’s value/maximum value of the laboratory range) values. Conclusions: Gender and thyroid volume seem to play an important role in the occurrence of thyroid toxicity, but further studies on dose– effect relationship for radiotherapy-induced thyroid toxicity are needed. © 2007 Elsevier Inc. Thyroid disorders, Radiotherapy, Cancer.