In hot areas, heat stress (HS) is one of the most prominent stressors in the broiler industry. To provide comprehensive information about the effect of N-acetyl cysteine (NAC) on hamartiology, biochemistry, and thyroid hormones in broilers under thermoneutral (TN) (22 4 ° C) and heat stress (HS) (34 2 ° C), The current study was carried out. A total of 80 one-day-old Cobb broiler chicks were used, with 20 birds in each group receiving either a control (basal diet) or a NAC (100 mg/Kg diet). Chronic HS at 42 days resulted in a considerable decrease in HGB, PCV, RBCs, total protein, Albumin, serum T3 and T4, while greatly increasing H/L, triglycerides, cholesterol, LDL-cholesterol, uric acid, AST, ALT, ALP, TBIL, DBIL, and creatinine. When compared to HSC broilers at 42 d old, NAC supplementation greatly increased PCV content with a substantial decrease in total leukocytic count, heterophils, and H/L ratio with an increase in lymphocytes and monocytes. When compared to HSC population, chickens given NAC had a significant decrease in serum AST, ALP, ALT and triglyceride levels, as well as a significant increase in serum T3 concentration.