The rhythmic, predictable fluctuation of hormones produced by the adrenal glands, primarily glucocorticoids like cortisol, over a 24-hour period. This cycling is a fundamental aspect of the body’s circadian system, essential for maintaining homeostasis and coordinating various physiological functions. Clinically, assessing this temporal pattern, rather than relying on a single point measurement, provides crucial insight into adrenal function and the body’s stress adaptation capacity.
Origin
The concept stems from the Greek ‘ad’ (near) and ‘renes’ (kidneys) for the gland location, and the term ‘hormone’ (to set in motion) combined with ‘cycling,’ referring to the established chronobiology of the endocrine system. The clinical relevance became prominent with the development of assays capable of measuring these temporal variations in steroid hormones. This terminology effectively bridges classical endocrinology with the modern science of chronobiology.
Mechanism
The cycling is primarily driven by the Hypothalamic-Pituitary-Adrenal (HPA) axis, which operates under the strict control of the Suprachiasmatic Nucleus (SCN), the body’s master clock. Corticotropin-releasing hormone (CRH) and Adrenocorticotropic hormone (ACTH) release exhibits a pulsatile pattern, leading to the characteristic cortisol peak shortly after waking and a subsequent nadir late at night. This precise mechanism ensures that energy metabolism, inflammation control, and immune responses are appropriately timed to the sleep-wake cycle.
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