Cortisol
Cortisol is a steroid hormone made by the adrenal glands. It helps regulate metabolism, immune activity, inflammation, blood pressure, sleep-wake timing, and the body's response to physical and psychological stress.
What cortisol is
Cortisol is a glucocorticoid hormone produced by the adrenal cortex. It is often called a stress hormone, but that nickname is incomplete. Cortisol helps the body manage energy, inflammation, immune activity, blood pressure, alertness, and adaptation to changing demands throughout the day.
The HPA axis
Cortisol release is controlled by the hypothalamus, pituitary gland, and adrenal glands. The hypothalamus sends signals that cause the pituitary to release ACTH, or adrenocorticotropic hormone. ACTH tells the adrenal cortex to make cortisol. Cortisol then feeds back to the brain and pituitary to help regulate further release.
Daily rhythm
Cortisol normally follows a circadian pattern. Levels are often higher in the morning, helping the body wake and mobilize energy, and lower later in the day and overnight. Sleep timing, shift work, illness, stress, medications, and some endocrine conditions can affect this rhythm.
Metabolism and energy
Cortisol helps make energy available during fasting, illness, exercise, or stress. It influences glucose production in the liver, protein breakdown, fat metabolism, and how tissues respond to other hormones. These effects are useful in short-term adaptation, but long-term excess cortisol can contribute to metabolic problems.
Inflammation and immunity
Cortisol helps restrain inflammation and immune activity. This is one reason synthetic glucocorticoid medicines can reduce inflammation in conditions such as asthma, autoimmune disease, allergic reactions, or transplant care. The same immune-suppressing effect can also raise infection risk or cause side effects when exposure is high or prolonged.
Low cortisol
Too little cortisol can occur when the adrenal glands, pituitary gland, or hypothalamus do not produce or signal properly. Adrenal insufficiency can cause fatigue, weakness, weight loss, low blood pressure, low blood sugar, nausea, salt craving, or dangerous adrenal crisis during stress. Diagnosis and treatment require medical testing and supervision.
High cortisol
Too much cortisol over time can occur from the body producing excess cortisol or from glucocorticoid medicines. Cushing syndrome is one medical pattern of high cortisol exposure. Possible features include weight gain in certain patterns, skin changes, muscle weakness, high blood pressure, high blood glucose, bone loss, mood changes, and infection risk.
Testing cortisol
Cortisol can be measured in blood, saliva, or urine depending on the question. Timing matters because levels change across the day. A single result can be misleading without context, medications, symptoms, illness, sleep schedule, and sometimes stimulation or suppression testing. Cortisol tests are tools for specific medical questions, not general stress scores.
Stress hormone, not stress score
Cortisol rises in many forms of physical and psychological stress, but it is not a simple measure of how stressed a person feels. Levels can change with sleep, infection, exercise, pain, pregnancy, medications, endocrine disease, and sampling time. Popular claims about cortisol often ignore how carefully hormone results must be interpreted.
Why it matters
Cortisol matters because it helps the body coordinate energy, immunity, circulation, and timing under changing conditions. Understanding cortisol clarifies why adrenal disorders can be serious, why steroid medicines need careful use, why sleep and stress biology are connected, and why hormone testing must be interpreted with care.