Adrenal Cortex: Function, Hormones, and Layers

The adrenal cortex is the outer shell of each adrenal gland, and its job is to produce steroid hormones that regulate blood pressure, metabolism, immune responses, and sexual development. It sits like a cap on top of each kidney, wrapped around an inner core called the medulla. The cortex is organized into three distinct cellular layers, each manufacturing a different class of hormone from the same raw material: cholesterol. That shared starting ingredient, combined with slightly different enzyme toolkits in each layer, accounts for the surprisingly wide range of hormones a single tissue can produce.

Three Layers, Three Hormone Classes

From the outside in, the three zones of the adrenal cortex are the zona glomerulosa, the zona fasciculata, and the zona reticularis. Each has a recognizable appearance under a microscope and a specific hormonal output. The zona glomerulosa, the thinnest outer rim, is made up of tightly packed cell clusters that produce mineralocorticoids, chiefly aldosterone. The zona fasciculata is the widest middle band, composed of large lipid-rich cells arranged in neat columns; it produces glucocorticoids, primarily cortisol in humans. The zona reticularis, the innermost layer bordering the medulla, contains compact, pigment-laden cells that produce adrenal androgens.¹²

The lipid droplets visible in the zona fasciculata cells are stores of cholesterol esters, ready to be converted into cortisol on demand. The zona reticularis cells, by contrast, look darker because they accumulate lipofuscin pigment as a byproduct of their particular enzymatic activity. These visual differences are not just cosmetic; they reflect genuinely different biochemical environments inside each zone, even though all three share the same starting molecule.

How Cholesterol Becomes a Hormone

Every steroid hormone the adrenal cortex produces begins as cholesterol. The rate-limiting step is not a chemical reaction but a physical one: moving cholesterol from the outer membrane of the mitochondria to the inner membrane, where the enzyme machinery lives. A protein called StAR (steroidogenic acute regulatory protein) handles that transport.³ Once cholesterol reaches the inner membrane, an enzyme clips its side chain to form pregnenolone, the first steroid in the pathway. From pregnenolone, a cascade of different enzymes in each zone channels the molecule toward aldosterone, cortisol, or androgens depending on which enzymes that zone expresses.⁴

Think of it like a highway that branches at several exits. The on-ramp (StAR moving cholesterol) is the same for everyone; the exit each cell takes depends on its local enzyme toolkit. The zona glomerulosa cells have the enzymes to make aldosterone but lack the one needed for cortisol. The zona fasciculata cells have what it takes to build cortisol but not aldosterone. This division of labor is remarkably clean, though not absolutely watertight: small amounts of crossover can occur, and disrupting one zone’s enzymes can redirect precursors into unexpected products, as happens in some genetic conditions.

Aldosterone and Blood Pressure Control

Aldosterone, the main mineralocorticoid, acts on the kidneys to retain sodium and excrete potassium. When sodium is held onto, water follows it, which raises blood volume and blood pressure. At the cellular level, aldosterone binds to receptors inside kidney tubule cells and switches on genes that increase the number of sodium channels on the cell surface, letting more sodium flow back into the body.⁵ This process also has downstream effects on potassium balance, since sodium reabsorption and potassium secretion are coupled in the kidney’s distal tubule.⁶

The body does not leave aldosterone production to chance. It is tightly controlled by the renin-angiotensin-aldosterone system (RAAS). When blood pressure drops or sodium levels fall, the kidneys release renin, which triggers a chain of reactions that produces angiotensin II. Angiotensin II then stimulates the zona glomerulosa to secrete aldosterone. A rise in blood potassium can also directly stimulate aldosterone release, independent of RAAS. When researchers block angiotensin II with antagonist drugs, the aldosterone response to angiotensin II shuts down, but the response to potassium remains intact, confirming these are two separate triggers.⁷ Other signaling molecules can fine-tune the system as well; for example, the neuropeptide neurotensin can dial down the zona glomerulosa’s response to both angiotensin II and potassium.⁸

Cortisol and the Stress Response

Cortisol, the zona fasciculata’s flagship product, does far more than respond to stress, though that is what it is best known for. It raises blood sugar by ramping up gluconeogenesis, the liver’s process of building new glucose from non-sugar precursors. In one study, a high-dose cortisol infusion over four hours increased glucose production entirely through gluconeogenesis.⁹ Cortisol also suppresses inflammation, slows down immune activity, helps maintain blood pressure, and influences mood and cognition. These effects are useful in the short term but damaging if cortisol stays elevated for weeks or months.

Cortisol secretion is governed by the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus releases corticotropin-releasing hormone (CRH), which prompts the pituitary gland to release adrenocorticotropic hormone (ACTH), which in turn tells the zona fasciculata to produce cortisol. When cortisol levels rise high enough, cortisol feeds back to the hypothalamus and pituitary to suppress further CRH and ACTH release, shutting off its own production. This negative feedback loop keeps cortisol within a healthy range under normal conditions.

The Daily Rhythm of Cortisol

Cortisol is not produced at a steady rate throughout the day. It follows a strong circadian pattern: levels peak in the early morning, within about 30 to 45 minutes of waking, and gradually decline through the afternoon and evening, reaching their lowest point around midnight. This daily slope matters for health. A meta-analysis found that people whose cortisol curve was flatter than normal, meaning it did not drop off properly during the day, had worse outcomes across a range of health measures, with the strongest links seen for immune and inflammatory problems and for cancer progression.¹⁰

This circadian rhythm has practical implications for how synthetic glucocorticoids are prescribed. Drugs like prednisone and dexamethasone mimic cortisol’s actions but can throw off the HPA axis if dosed at the wrong time or for too long. Interest is growing in timing glucocorticoid doses to align with the body’s natural rhythm, minimizing HPA axis suppression while preserving the anti-inflammatory effects clinicians are after.¹¹ Anyone who has taken a steroid taper knows the drill of morning-only dosing; that scheduling choice reflects this biology.

Adrenal Androgens and Adrenarche

The zona reticularis produces androgens, most prominently dehydroepiandrosterone (DHEA) and its sulfated form, DHEA-S. These are sometimes called “weak” androgens because they have modest direct effects, but the body converts them into more potent androgens and even estrogens in peripheral tissues. In women, the adrenal cortex is a major source of androgens because the ovaries produce comparatively small amounts, so adrenal androgen output plays a more prominent role in female physiology than many people realize.

The most visible moment in adrenal androgen biology is adrenarche, a developmental event that starts around age six. The zona reticularis expands and begins expressing a distinctive enzyme profile: higher levels of cytochrome b5 and steroid sulfotransferase, with lower levels of another enzyme that would otherwise divert precursors away from DHEA.¹² The result is a surge in DHEA and DHEA-S production. By about age eight, this rise becomes externally visible as the appearance of pubic and axillary hair in both boys and girls, a milestone called pubarche.¹³ Adrenarche is distinct from puberty; it is driven by the adrenal cortex rather than by gonadal hormones, and it starts earlier. The two processes overlap in timing but have separate regulatory mechanisms.

The Fetal Adrenal Cortex

Before birth, the adrenal cortex looks quite different from its adult version. The fetal adrenal has a large inner zone, aptly called the fetal zone, which produces enormous quantities of DHEA. This output is not for the fetus’s own use; instead, the placenta converts it into estrogen, which supports the pregnancy. The fetal zone supplies roughly 60% of the DHEA used for placental estrogen production, and placental estrogen in turn shapes how the fetal adrenal cortex develops, creating a feedback loop between the two organs.¹⁴ After birth, the fetal zone rapidly shrinks and is replaced by the three-zone adult architecture over the first months of life.

The Cortex Talks to the Medulla

The adrenal cortex and the adrenal medulla are often discussed as though they are two unrelated organs that happen to share an address. In reality, the cortex directly influences medullary function. Blood drains from the cortex inward through a portal vascular system, bathing the medulla in cortisol concentrations far higher than what the rest of the body sees. The medulla needs those high cortisol levels to produce an enzyme called PNMT, which converts norepinephrine into epinephrine (adrenaline).¹⁵ Without cortisol flowing in from the surrounding cortex, the medulla would produce norepinephrine but struggle to make epinephrine. This means the cortex’s hormonal output has a direct hand in shaping the body’s acute fight-or-flight chemistry.

When the Cortex Underperforms

Primary adrenal insufficiency, historically called Addison’s disease, occurs when the adrenal cortex itself is damaged or fails. In adults, the most common cause is autoimmune destruction, where the immune system attacks cortical tissue. In children, genetic enzyme defects are more frequently to blame, and the spectrum of known genetic causes has expanded as sequencing technology improves.¹⁶ The hallmark of adrenal insufficiency is a shortage of cortisol and often aldosterone, leading to fatigue, low blood pressure, salt cravings, and, in crisis situations, dangerously low blood sugar and circulatory collapse. People with this condition rely on daily hormone replacement.

A more specific genetic condition, congenital adrenal hyperplasia (CAH), illustrates what happens when a single enzyme in the steroid pathway is missing. The most common form results from a deficiency in 21-hydroxylase, which blocks both cortisol and aldosterone synthesis. Because cortisol cannot be made, the HPA axis never receives its “stop” signal, so ACTH levels keep climbing. The constant ACTH drive enlarges the adrenal cortex (hence “hyperplasia”) and pushes precursor molecules down the only remaining open pathway: androgen production. The excess androgens can accelerate bone maturation and cause early growth plate closure, as well as virilization in affected girls.¹⁷¹⁸ Treatment involves replacing the missing cortisol, which dials ACTH back down and reduces the androgen overflow.

When the Cortex Overproduces

The opposite problem, too much cortisol, leads to Cushing syndrome. The classic physical signs include a round face, fat accumulation at the back of the neck, reddish-purple stretch marks on the abdomen, and excess hair growth. The excess cortisol can come from a pituitary tumor overproducing ACTH, an adrenal tumor making cortisol autonomously, or, more rarely, a tumor elsewhere in the body secreting ACTH ectopically.¹⁹ Distinguishing among these causes can be genuinely tricky, since pituitary incidentalomas (small, non-functioning tumors found by accident on imaging) are common and can confuse the diagnostic workup when the real source is elsewhere.

Overproduction of aldosterone, called primary aldosteronism, is a common cause of high blood pressure that often goes unrecognized. It may be caused by a small benign tumor in the zona glomerulosa (an aldosterone-producing adenoma) or by bilateral overgrowth of that zone. Surgery can cure the unilateral form, though recurrence has been documented even more than a decade later in rare cases.²⁰ When the cause is bilateral, lifelong medication with a mineralocorticoid receptor blocker like spironolactone is the standard approach. Pinpointing which adrenal is the culprit often requires adrenal vein sampling, an invasive procedure where blood is drawn directly from the veins draining each adrenal gland to measure aldosterone levels on each side.²¹

An Evolutionary Perspective on the Cortex

The neat cortex-on-the-outside, medulla-on-the-inside arrangement of the mammalian adrenal gland is not the default across the animal kingdom. In fish, the two tissue types exist as separate cell clusters with no shared capsule at all. Reptiles represent an intermediate state: steroidogenic and chromaffin (adrenaline-producing) tissues are closely associated but variably mixed, often sitting near the gonads rather than on top of the kidneys. In squamate reptiles like lizards and snakes, the degree of mixing between the two tissue types and the ratio of norepinephrine to epinephrine produced by the chromaffin cells vary considerably from one species to another.²² The fully layered, encapsulated adrenal gland is a mammalian innovation, and as described earlier, its architecture enables the cortex-to-medulla blood flow that mammals depend on for efficient epinephrine synthesis.

Gut Bacteria and Adrenal Function

One of the more unexpected frontiers in adrenal research involves the gut microbiome. A Mendelian randomization study found evidence for a causal relationship between specific gut bacterial populations and adrenal function, adding a new dimension to the gut-brain axis concept.²³ Separately, gut bacteria have been found to possess their own steroid-processing enzymes capable of cleaving the side chain of cortisol and its derivatives to produce potent androgens called 11-oxy-androgens. This means microbes in the intestine can directly modify steroid hormones that the adrenal cortex produces, acting almost as an additional steroidogenic compartment.²⁴ The clinical significance of microbial steroid metabolism is still being worked out, but it raises the possibility that changes in your gut flora could influence how much active androgen or glucocorticoid your body ends up seeing, regardless of what your adrenal glands actually secrete.