Jennifer Ezell, ACNP, CEN, MSN, replies: To understand the rationale for hydrocortisone, you need to recall the normal physiology and function of the adrenal glands, which secrete both mineral-corticoids (primarily aldosterone) and glucocorticoids (primarily cortisol).
The secretion of glucocorticoids is regulated through the hypothalamic-pituitary-adrenal (HPA) axis, which simply functions as a negative feedback loop. Low levels of serum cortisol stimulate the hypothalamus to secrete corticotropin-releasing hormone, which promotes the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. The ACTH acts on the adrenal cortex to increase glucocorticoid secretion.
Maintaining cortisol production is crucial to health. Cortisol is a prerequisite for the production of angiotensin II, which maintains vascular tone and normal blood pressure. Cortisol also counters the effects of insulin (preventing hypoglycemia), increases free water clearance, inhibits the inflammatory response, and lowers serum calcium by inhibiting its uptake in the renal tubule and gut and by redistributing calcium to the intracellular spaces.
When a patient takes a glucocorticoid such as prednisone for a long period, the body makes less endogenous glucocorticoid. The adrenal cortex atrophies and the normal feedback loop (the HPA axis) is turned off.
In your patient's case, suddenly stopping prednisone reduced her glucocorticoid level dramatically, and her depressed system had no chance to compensate. This is called secondary adrenal insufficiency because the problem didn't originate with the adrenal glands, but with an outside source (in this case, the prednisone the patient had been taking).
Treatment involves administering an exogenous form of cortisol (hydrocortisone) to correct the immediate crisis and restore homeostasis. Should the patient then need to stop the hydrocortisone or prednisone, she must be weaned slowly so that the HPA axis has time to kick in again.
Teach all patients taking long-term steroids such as prednisone to renew their prescriptions promptly so they don't run out of medicine, and never to abruptly stop the drug for any reason.