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Cortisol is a steroid hormone released from the adrenal cortex in response to a hormone called ACTH (produced by the pituitary gland). Normally, cortisol levels rise and fall during the day, repeating a 24-hour cycle (diurnal variation). Highest levels are at about 6-8 a.m. and lowest levels are at about midnight.
Physical and emotional stress can increase serum cortisol, because a normal response to stress involves increased secretion of ACTH by the pituitary gland.
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