Introduction: Adjuvant radiotherapy is considered one of treatment options for treatment of locally advanced breast cancer. Thyroid gland is not considered an organ at risk in supraclavicular (SC) nodal radiation therapy (RT) for breast cancer. Objectives: Comparison the impact of 2 different RT techniques on thyroid gland; group (I) conventional two-dimensional radiotherapy of SC node and Group (II) three-dimensional conformal radiation therapy (3DCRT) planning of SC node. Method: Twenty (20) patients with breast cancer received SC RT, with evaluation of  thyroid functions in both groups, including thyroid stimulating hormone and  free thyroxine prior to RT and 3, 6, 9, 12, 18 and 24 months after RT. Based on each patient's dose volume histogram (DVH),total volume of thyroid and mean radiation dose of the thyroid which received radiation doses 10-50 Gy (V10-V50) were considered for statistical analysis. The median follow-up time was 20 months (range, 12-30 months). Results: of 20 patients, 5 (20%) were diagnosed with hypothyroidism (HT), 4 group (II) and 1 group (I). The median time to the development of HT was 9 months. SC node V50 in group I was lower than average in group II (Average: 2.06% versus 55.38% p<0.001).Thyroid V20 in group I was lower than average in group II (Average: 37.65% versus 50.47%, p<0.001) mean TSH in group I was lower than mean in group II (Average: 1.60 IU/L versus 3.08 IU/L, p=0.043). Conclusion: Conformal supraclavicular RT in patients with breast cancer appear to amplify the risk of HT more than conventional, however conformal radiotherapy is better than conventional radiotherapy in locoregional control