Purpose: Thyroid gland is one of the radiosensitive endocrine organs in the body. It has been shown that direct irradiation of thyroid with total doses of 26 to 30Gy can lead to functional abnormalities. In this study, irradiation doses on thyroid gland of the patients who received postoperative chest-wall/breast and regional nodal irradiation were assessed. Materials and Methods: Retrospective analyses of treatment plans from 122 breast cancer patients who were treated with 3D conformal radiotherapy (3D CRT) planning was performed. All patients received irradiation to supraclavicular/level III lymph nodes in addition to chest-wall/breast. A total dose of 46Gy was delivered in 25 days to supraclavicular/level III lymph node region while a total dose of 50Gy was delivered to whole breast/chest-wall. Thyroid gland was contoured on 2-5 mm thickness of computed tomography scans. Absolute thyroid volume, mean thyroid doses were calculated. Results: The mean thyroid volume of all patients was 16.7 cc (min: 1.9 cc, max: 41.6 cc). The mean irradiation dose on was 22.5 Gy (0.32 Gy - 46.5 Gy). The level of dose was higher than 26 Gy in 44% of the patients. Conclusions: In majority of the node-positive breast cancer patients treated with 3D CRT, the thyroid gland was exposed to considerable doses. On the other hand, for 44% of the patients are at risk for developing thyroid function abnormalities which should be considered during the routine follow-up.