Reference library
Plain-language definitions. Careful boundaries. Sources you can check.
Showing all 18 terms
Quick index
Medical, research, health-data, and product language are labeled so you always know what kind of definition you are reading.
Adrenocorticotropic hormone
A hormone made by the pituitary gland. ACTH signals the adrenal glands to make cortisol.
Why it matters here ACTH is part of the body’s feedback system for adjusting cortisol production. It is measured with a clinical blood test—not by Apple Watch.
A severe, potentially life-threatening worsening of adrenal insufficiency. It needs immediate medical treatment.
Safety note If you have diagnosed adrenal insufficiency, follow your clinician’s emergency plan and seek immediate care for possible crisis symptoms. Call your local emergency number.
Two small glands that sit above the kidneys. They make cortisol, aldosterone, and other hormones the body needs.
Why it matters here Cortisol is made in the adrenal glands, but its production is guided by signals from the brain and pituitary gland.
A clinician-diagnosed condition in which the body does not make enough of certain hormones, including cortisol.
Why the distinction matters Primary, secondary, and tertiary forms begin in different parts of the hormone signaling system. Symptoms alone cannot identify the cause.
Primary adrenal insufficiency caused by damage to the adrenal glands. Autoimmune disease is a common cause.
Keep the boundary clear Addison’s disease is a recognized medical condition. It is not the same as the unsupported idea called “adrenal fatigue.”
A roughly 24-hour pattern in physical, mental, and behavioral processes. Light and darkness are major cues for these rhythms.
Why it matters here Sleep timing, light exposure, and hormone patterns all have daily rhythms. A single reading cannot describe the whole pattern.
An essential hormone made by the adrenal glands. It helps the body respond to stress, reduce inflammation, regulate blood glucose and metabolism, and control blood pressure.
Keep the boundary clear Clinical cortisol testing uses blood, urine, or saliva. Cortisol Aware and Apple Watch do not measure or estimate cortisol.
A research term for the marked rise in cortisol secretion during roughly the first 30–45 minutes after waking.
Measurement matters Researchers use carefully timed biological samples, often saliva, beginning at waking. Wearable sleep or heart data does not measure this response.
Corticosteroids are medicines related to hormones made by the adrenal glands. They can reduce inflammation and affect the immune system.
Medication safety These are powerful medicines. Do not stop, start, or change a prescription corticosteroid based on this site or app. Talk with the clinician who prescribed it.
The form of Cushing’s syndrome caused by a pituitary tumor that makes too much ACTH, which then drives excess cortisol production.
Why the name matters Cushing’s disease is one cause of Cushing’s syndrome. The two names are related, but they are not interchangeable.
A clinician-diagnosed disorder caused by exposure to too much cortisol over a long period.
Causes differ Long-term use of high-dose corticosteroid medicine is a common cause. Some tumors can also cause the body to make too much cortisol. Diagnosis requires clinical testing.
Endogenous means originating inside the body. Exogenous means coming from outside the body.
In cortisol discussions Endogenous excess means the body is making too much cortisol. Exogenous exposure can come from corticosteroid medicine. Never change prescribed medicine without clinical guidance.
A chemical messenger made by endocrine glands. Hormones travel through the bloodstream to tissues or organs and help regulate many body processes.
Why it matters here Cortisol and ACTH are hormones. Their levels and effects cannot be inferred from one feeling or one wearable metric.
Hypothalamic–pituitary–adrenal axis
The connected hormone signaling system involving the hypothalamus, pituitary gland, and adrenal glands.
Why it matters here This feedback system helps regulate ACTH and cortisol. “Axis” describes a biological network, not a score that a watch can read.
Variation in the time between individual heartbeats. HealthKit records HRV using SDNN, the standard deviation of normal heartbeat intervals.
Keep the boundary clear HRV has many influences. It is not a cortisol test and cannot reveal a cortisol level.
In Cortisol Aware, a comparison between your current available observations and your own recent history.
What it is not A personal baseline is not a diagnosis, a laboratory reference range, a population “normal,” or proof that one action caused a change.
The number of heartbeats per minute while at rest. A resting pulse is measured after a period without activity.
Keep the boundary clear Resting heart rate has many influences. It does not measure cortisol or explain, by itself, why you feel a certain way.
The way the brain and body respond to a challenge or demand. It can include changes in alertness, hormones, heart rate, blood pressure, and blood glucose.
Why it matters here Stress is not always harmful, and it cannot be reduced to one hormone or one wearable reading. Cortisol Aware focuses on observable patterns and low-risk routines.
Try a shorter word, such as “heart,” “adrenal,” or “sleep.”
These definitions make the language easier to follow. They cannot tell you whether you have a medical condition. Persistent, worsening, or concerning symptoms deserve evaluation by a qualified health professional.
