Hypothalamic-Pituitary-Adrenal Axis
The physiological cascade of cortisol production is initiated in the central nervous system and completed in the periphery — the adrenal cortex. This process can be divided into several strictly sequential steps:
- In response to provoking stimuli, such as acute infection, trauma, or a critical drop in blood glucose concentration, hypothalamic structures release a specific releasing factor into the bloodstream — corticotropin-releasing hormone (CRH).
- CRH reaches the anterior pituitary gland, stimulating its cells to synthesize and secrete adrenocorticotropic hormone (ACTH).
- In turn, circulating ACTH acts on receptors in the adrenal cortex, directly activating the synthesis and subsequent secretion of cortisol.
Notably, cortisol is one of the counter-regulatory hormones. In metabolic hormonal regulation, it acts as a major physiological antagonist to insulin, alongside epinephrine, growth hormone (somatotropin), and iodothyronines (thyroid hormones).
Negative Feedback Mechanism
To prevent excessive depletion of the body's energy resources and the toxic effects of high glucocorticoid concentrations, the regulatory system employs a strict self-control mechanism. As soon as systemic blood cortisol levels rise significantly, it directly inhibits the upstream endocrine glands.
High cortisol suppresses:
- The synthesis and release of CRH in hypothalamic structures.
- The secretion of ACTH by pituitary cells.
Through this biological "brake," hormone levels smoothly return to baseline once the initial stressor is resolved.
Catabolism and Excretion of Corticosteroids
Spent corticosteroid hormones undergo chemical transformations for subsequent elimination from the body. The main products of corticosteroid catabolism (with the exception of compounds such as aldosterone and corticosterone) are transformed into 17-ketosteroids.
To ensure water solubility and successful urinary excretion, these metabolites form stable conjugates, primarily complexes with glucuronic and sulfuric acids.
Evaluating 27-ketosteroid levels in 24-hour urine is of great clinical significance, though interpretation depends on patient sex:
- In men, normal excretion ranges from 12 to 17 mg per day. About two-thirds (2/3) of this volume is derived from adrenal corticosteroid metabolites, while the remaining third (1/3) comes from testosterone derivatives synthesized by the testes.
- In women, reference values are noticeably lower: from 7 to 12 mg per day, and these excreted compounds are predominantly of adrenal origin.