The current study aimed to evaluate the effects of potassium iodide in-ovo injection on thyroid gland activity and some hematological and biochemical indices of broiler chicks. A total of four hundred seventy-seven fertilized broiler eggs (Ross 308) were randomly distributed into 3 groups (159 eggs/group), with 3 replicates (53 eggs/replicate). Eggs of 1 st group were considered as a negative control (not injected), the 2 nd group eggs were injected with 0.2 mL deionized water (positive control), whereas the 3 rd group eggs were injected into the allantoic sac, with 2 mg potassium iodide/ egg in 0.2 mL volume, on the 10 th day of incubation. On hatch day, hatchability%, embryonic mortality%, chicks weight were measured, and hematological and biochemical tests were conducted. The weight of the hatched chicks from injected eggs with potassium iodide was significantly lower than that in negative control group (P≤0.05), moreover, a significant increase was recorded for hemoglobin, packed cell volume%, lymphocytes%, and thyroxine concentration as compared with the control group, meanwhile a significant decrease was recorded for the heterophils%, H/L ratio, cholesterol, triglycerides and triiodothyronine concentration as compared with the control group (P≤0.05). In conclusion, potassium iodide has an important impact on thyroid activity and reduces stress on the incubated eggs, as well as on some hematological indices of hatched chicks.