Hypothalamic–pituitary–adrenal axis

The hypothalamic–pituitary–adrenal axis is a hormonal signaling system in which the hypothalamus and pituitary gland regulate cortisol release from the adrenal cortex, especially during stress.

Corticotropin-releasing hormone: A hypothalamic hormone that stimulates the anterior pituitary to release adrenocorticotropic hormone. It initiates the axis’s hormone cascade in response to neural and hormonal signals.
Stress response: The coordinated physiological and behavioral changes that help an organism respond to a challenge. The axis mobilizes energy and helps sustain bodily responses during stress.
Hypothalamus: A brain region that links neural activity with endocrine and autonomic regulation. Its paraventricular neurons release the signals that begin HPA-axis activation.
Cortisol awakening response: The rise in cortisol during the period shortly after waking in the morning. Its variability offers a measurable view of daily HPA-axis regulation.
Adrenocorticotropic hormone: A pituitary hormone that stimulates the adrenal cortex to produce and release cortisol. It carries the pituitary’s signal to the cortisol-producing part of the adrenal gland.
Hypothalamic–pituitary–adrenal axis in depression: The study of altered HPA-axis regulation associated with some forms of depression. Changes in cortisol signaling are a prominent, though variable, finding in depression research.
Pituitary gland: An endocrine gland beneath the hypothalamus that releases hormones under neural and hormonal control. Its anterior lobe releases ACTH in response to hypothalamic signaling.
Diurnal cortisol rhythm: The predictable daily pattern of cortisol concentrations, typically highest near waking and lowest around midnight. The rhythm reflects clock-time influences on axis activity beyond immediate stressors.
Cortisol: A steroid hormone made by the adrenal cortex that affects metabolism, immune activity, and stress responses. It is the axis’s principal output and feeds back to suppress upstream signaling.
Dexamethasone suppression test: A test that uses synthetic glucocorticoid dexamethasone to assess suppression of cortisol production. It probes whether glucocorticoid feedback can suppress the axis’s cortisol output.

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