Abstract Aim Hypothyroidism is not commonly considered as a complication of radiotherapy to the head and neck region. The purpose of this retrospective study was to determine the frequency of thyroid dysfunction in patients after radiotherapy, to compare thyroid hormone levels in irradiated patients with Control group and to predict development of thyroid hypofunction in time. Material and method Thyroid function was measured by means of thyroid stimulating hormone (TSH), free thyroxine (FT4) and free triiodthyronine (FT3) in 43 patients who had nonthyroid head–neck carcinomas treated by radiotherapy or radiotherapy in combination with other modalities. These data were compared with hormone levels of 40 Control group patients treated solely by surgery. Results Hypothyroidism was found in 35% of irradiated patients. In comparison with Control group there were significant differences between TSH and FT4 levels, difference between FT3 levels was insignificant. A correlation between hormone levels and follow-up was detected. Conclusion Our results indicate that hypothyroidism is a frequent late side effect of radiotherapy to head and neck. Lifelong monitoring of thyroid function appears to be justified when radiotherapy is a part of treatment protocol.