Impairment of thyroid function has been described in up to 50% of the patients after external irradiation of the neck region as well as after mantle irradiation. In order to assess radiation-induced alterations in cultured thyroid cells, the occurrence of apoptosis and necrosis as well as the expression of thyroid peroxidase (TPO) and of two members of the 70 kD heat shock family, HSP-73 and HSP-72, were analysed following gamma irradiation. Human thyroid epithelial cells (TEC) were purified from surgical tissue specimens, were cultured and irradiated with a single dose of 5 Gy or 50 Gy using Co60 as radioactive source. Analysis was performed 1, 3 and 5 day(s) after irradiation. Apoptosis and necrosis were assessed by DNA staining with propidium iodide and FACS analysis. TPO and HSP expression by SDS-PAGE and Western blotting. The cell viability of TEC was not affected by irradiation and there was no induction of HSP-72, a sensitive indicator of acute cellular stress. Interestingly, the expression of TPO, a key enzyme of thyroid hormone synthesis, decreased significantly in irradiated TEC, while HSP-73 expression remained unchanged. Decreased expression of TPO with a resulting suppression of thyroid hormone synthesis could contribute to an early development of thyroid dysfunction following irradiation. Thus, analysis of thyroid function, even early after external radiation therapy of the neck or after total body irradiation, seems to be indicated.