Cortisol

Cortisol is a glucocorticoid hormone produced by the adrenal cortex as the end product of the HPA axis. Its relationship with Mast Cells is paradoxical: in pharmacological doses, corticosteroids suppress mast cell activity — but the stress-cortisol system as a whole can activate mast cells through CRH.

The Paradox

Pharmacological cortisol (prednisone, dexamethasone): At high doses, corticosteroids are powerful immunosuppressants. They reduce mast cell mediator production, suppress Cytokines, and inhibit Degranulation. This is why a burst of prednisone can dramatically improve MCAS flares.

Physiological cortisol (endogenous stress response): The problem isn’t cortisol itself — it’s the CRH that triggers cortisol production. CRH directly activates mast cells via CRH Receptors before cortisol even enters the picture. And in chronic stress / PTSD, the cortisol feedback loop is impaired, meaning CRH stays elevated longer.

Circadian Pattern

Cortisol follows a diurnal rhythm — lowest around midnight, rising from approximately 3-4 AM, peaking around 8 AM. This cortisol rise coincides with a peak in mast cell activity, which is one reason symptoms can be worst on waking. See Sleep and Histamine.

Chronic Cortisol Dysregulation

In PTSD and chronic stress, the HPA axis is reset to a higher baseline of CRH production with impaired cortisol negative feedback. This creates a state where mast cells are chronically stimulated by stress hormones even in the absence of an acute stressor. The “always on” quality of chronic stress literally means “always activating mast cells.” See The HPA Axis and Mast Cells.