Does stress-axis activation shift adrenal steroidogenesis toward cortisol?

Biochemical pathways of adrenal steroidogenesis

• Progesterone as a central precursor: Progesterone is an essential, biochemically established intermediate in adrenal steroidogenesis. Synthesized from cholesterol via pregnenolone in the inner mitochondrial membrane, it serves as a branching precursor. In the zona glomerulosa, it is hydroxylated by 21-hydroxylase (CYP21A2) toward aldosterone. In the zona fasciculata, it is hydroxylated by CYP17A1 into 17-hydroxyprogesterone, which then undergoes subsequent enzymatic conversion via CYP21A2 and 11-hydroxylase (CYP11B1) to yield cortisol.

Stress-axis activation and precursor dynamics

• Local upregulation vs. "precursor siphoning": The traditional concept of a physical "pregnenolone steal" or competitive siphoning of a shared, body-wide precursor pool during stress is biochemically incorrect. Adrenal cells do not share or exchange mitochondrial precursor pools. However, stress-axis activation does preferentially increase cortisol production. Hypothalamic-pituitary-adrenal (HPA) axis activation releases adrenocorticotropic hormone (ACTH), which selectively upregulates cholesterol transport and enzymatic machinery (such as CYP11A1 and CYP17A1) locally within the mitochondria of the zona fasciculata.

Clinical relevance of diurnal cortisol rhythms

• Impaired glucose regulation: Cortisol is a potent insulin antagonist that stimulates hepatic gluconeogenesis and decreases glucose uptake in peripheral tissues like skeletal muscle and adipose tissue. Disrupted cortisol rhythms, particularly elevated evening cortisol or a flattened diurnal slope, are directly linked to hyperglycemia and insulin resistance.
• Fatigue and low energy: A healthy diurnal cortisol pattern features a distinct morning peak (the cortisol awakening response) followed by a gradual decline. Flattened diurnal curves—characterized by a blunted morning rise and high evening levels—are strongly correlated with persistent fatigue, sleep disruption, and decreased physical energy, as seen in population studies of chronic fatigue states.

Bottom line

While stress does not literally "steal" a shared pool of progesterone, HPA-axis activation selectively drives cortisol production. Evaluating unmeasured diurnal cortisol rhythms is highly clinically relevant in patients presenting with concurrent low energy and elevated glucose, as a flattened cortisol curve is a shared biological driver of both fatigue and glycemic instability.