The normal range of blood cortisol varies according to the time of measurement. The average value of blood cortisol at 8:00 a.m. in normal adults is (276±66) nmol/L (range 165-441 nmol/L). Cortisol is synthesized and secreted by the adrenal cortex, and the measurement of serum cortisol can reflect the secretory function of the adrenal cortex; depending on the time of the measurement, the normal range of cortisol will also vary. The circadian rhythm of plasma cortisol: the average value at 8:00 a.m. in normal adults is (276±66) nmol/L (range 165-441nmol/L); the average value at 4:00 p.m. is (129.6±52.4) nmol/L (range 55-248nmol/L); the average value at 12:00 p.m. is (96.5±33.1) nmol/L (range 55-138nmol/L). 138 nmol/L. Increased serum cortisol can be seen in adrenal hyperplasia and tumors; decreased serum cortisol can be seen in adrenocortical hypoplasia and hypopituitarism. It is recommended to go to the hospital when abnormalities in serum cortisol occur.