Cushing's Syndrome: Causes, Symptoms, Diagnosis, and Treatment

What Is Cushing's Syndrome?

Cushing's syndrome is the clinical result of prolonged, pathological exposure to excess glucocorticoids, whether from an internal source or from prescribed medication. The condition was first described by neurosurgeon Harvey Cushing in 1912. Endogenous cases affect roughly 10 to 15 people per million per year, though subclinical cortisol excess detected on adrenal imaging may be considerably more prevalent [1].

Cortisol is produced by the zona fasciculata of the adrenal cortex. Under normal physiology, the hypothalamus releases corticotropin-releasing hormone (CRH), which triggers the anterior pituitary to secrete adrenocorticotropic hormone (ACTH), which then drives cortisol synthesis. Chronic disruption at any point in this axis produces either too much or too little cortisol.

Understanding the distinction between Cushing's syndrome and Cushing's disease matters for treatment planning. Cushing's syndrome is the umbrella term for any source of cortisol excess. Cushing's disease refers specifically to a pituitary corticotroph adenoma that secretes excess ACTH. According to the Endocrine Society's 2008 clinical practice guideline, "Cushing's disease accounts for approximately 70% of endogenous ACTH-dependent Cushing's syndrome" [2].

The opposite end of the cortisol spectrum is Addison's disease, also called primary adrenal insufficiency, where the adrenal glands produce too little cortisol and aldosterone. Addison's affects roughly 100 to 140 per million people and is discussed further in the section on adrenal insufficiency below [3].

What Causes Cushing's Syndrome?

Causes split cleanly into two categories: exogenous (iatrogenic) and endogenous. Exogenous glucocorticoid use is by far the most common cause globally.

Exogenous causes. Chronic use of prednisone, dexamethasone, or any systemic glucocorticoid at supraphysiologic doses suppresses the HPA axis and reproduces the full clinical picture of Cushing's syndrome. Even inhaled or topical steroids can cause iatrogenic Cushing's in susceptible patients, particularly children, with prolonged high-dose use [4].

Endogenous ACTH-dependent causes (about 80% of endogenous cases).

• Cushing's disease: A pituitary corticotroph adenoma drives ACTH excess, which stimulates bilateral adrenal hyperplasia and cortisol overproduction.
• Ectopic ACTH syndrome: Small-cell lung carcinoma, bronchial carcinoid tumors, and pancreatic neuroendocrine tumors secrete ACTH autonomously. This variant typically presents with severe hypokalemia and rapid onset rather than the more gradual weight gain pattern seen in pituitary disease.

Endogenous ACTH-independent causes (about 20% of endogenous cases).

• Adrenal adenoma: A benign tumor secreting cortisol autonomously.
• Adrenal carcinoma: Rare, often large, and may co-secrete androgens or mineralocorticoids.
• ACTH-independent macronodular adrenal hyperplasia (AIMAH): Both adrenal glands enlarge and produce cortisol independent of ACTH [5].

A 2020 analysis published in the European Journal of Endocrinology found that patients with ectopic ACTH syndrome had a significantly shorter delay to diagnosis (median 0.5 years) compared to Cushing's disease (median 2.4 years), reflecting the more dramatic biochemical picture in the ectopic form [6].

Symptoms and Clinical Signs of Cushing's Syndrome

The classic physical features are recognizable but develop gradually, which is why diagnosis is delayed an average of two to six years from symptom onset.

Cardinal features with high specificity.

• Centripetal obesity with supraclavicular and dorsocervical fat pads ("buffalo hump")
• Wide (greater than 1 cm) purple or violaceous striae on the abdomen, thighs, or breasts
• Proximal muscle weakness making it difficult to rise from a chair or climb stairs
• Easy bruising with thin, fragile skin

Common but nonspecific features.

• Hypertension (occurs in roughly 80% of patients) [7]
• Glucose intolerance or frank type 2 diabetes
• Osteoporosis and vertebral fracture risk
• Menstrual irregularity, hirsutism, or clitoral enlargement in women
• Decreased libido and erectile dysfunction in men
• Depression, anxiety, cognitive impairment, and sleep disturbance
• Recurrent or unusual infections from cortisol-mediated immune suppression

Children with Cushing's syndrome almost always show growth failure combined with weight gain. That combination, in any child, warrants cortisol evaluation.

The Endocrine Society guideline specifies four high-risk populations who warrant screening even without the full clinical picture: people with adrenal incidentalomas, children with decreasing height percentile plus increasing weight, patients with early-onset osteoporosis, and those with type 2 diabetes and hypertension before age 40 [2].

How Is Cushing's Syndrome Diagnosed?

Diagnosis requires biochemical confirmation across at least two different tests before any imaging is obtained. Imaging without biochemical proof leads to false attribution of adrenal or pituitary incidentalomas.

Step 1: Screening tests (choose two or more).

1. 24-hour urinary free cortisol (UFC): Values greater than three times the upper limit of normal on two separate collections are strongly suggestive. Simple elevation in the normal-high range is not diagnostic because obesity and depression can also raise UFC modestly [2].
2. Late-night salivary cortisol (LNSC): Cortisol nadir normally occurs between 11 PM and midnight. Two elevated LNSC values (using center-specific cutoffs) carry a sensitivity of approximately 92 to 100% and a specificity of 93 to 100% for Cushing's syndrome [8].
3. 1 mg overnight dexamethasone suppression test (DST): 1 mg dexamethasone at 11 PM; serum cortisol drawn at 8 AM. Suppression to below 1.8 mcg/dL essentially rules out the syndrome. Failure to suppress is sensitive but not specific, as many medications (rifampin, phenytoin, carbamazepine) increase dexamethasone metabolism and produce false positives [2].

Step 2: Establish ACTH dependence.

Once two screening tests confirm hypercortisolism, measure a morning plasma ACTH. A level below 5 pg/mL with a suppressed CRH stimulation response indicates ACTH-independent disease, directing attention to adrenal imaging. A level above 15 pg/mL indicates ACTH-dependent disease, directing attention to pituitary MRI [5].

Step 3: Localize the source.

• Pituitary MRI with gadolinium: Identifies a discrete adenoma in only 50 to 60% of confirmed Cushing's disease cases because many corticotroph adenomas are microadenomas smaller than 6 mm [2].
• Inferior petrosal sinus sampling (IPSS): When MRI is negative or equivocal, IPSS with CRH stimulation remains the gold standard for distinguishing pituitary from ectopic ACTH secretion, with a sensitivity of 88 to 100% and specificity approaching 100% [9].
• CT of chest, abdomen, and pelvis: Ordered when ectopic ACTH is suspected based on IPSS results or clinical context.

Cushing's Disease: The Pituitary-Specific Problem

Cushing's disease deserves its own discussion because it is the most common endogenous cause and because treatment decisions are nuanced. The typical corticotroph adenoma is a microadenoma (less than 10 mm), often invisible on standard MRI. Patients are most commonly women in their third to fourth decade, though the condition occurs across all ages and in men.

Transsphenoidal adenomectomy by an experienced pituitary neurosurgeon produces remission in 65 to 90% of microadenoma cases and 40 to 50% of macroadenoma cases [10]. Post-surgical remission is defined as a morning serum cortisol below 2 mcg/dL within the first post-operative week, which reflects successful removal of the autonomous ACTH source. The remaining healthy corticotrophs are suppressed after years of excess cortisol feedback, so patients require hydrocortisone replacement, typically 10 to 12 mg/m2/day divided into two doses, until their HPA axis recovers. Recovery may take 6 to 18 months [2].

Recurrence occurs in roughly 15 to 25% of patients within ten years, making long-term follow-up with annual late-night salivary cortisol measurements standard practice [10].

When surgery fails or is not possible, second-line options include repeat surgery, stereotactic radiosurgery (Gamma Knife), bilateral adrenalectomy, or medical therapy. Pasireotide (a somatostatin analogue) at 0.6 to 0.9 mg subcutaneously twice daily reduced 24-hour UFC to normal in 26% of patients in a Phase 3 trial of 162 participants [11]. Cabergoline, a dopamine agonist, normalizes UFC in roughly 30 to 40% of patients in small series [12]. These are not strong response rates, which is why surgery remains the preferred first step.

Medical Therapy for Cortisol Excess

Steroidogenesis inhibitors work by blocking the enzymatic steps that convert cholesterol to cortisol in the adrenal gland. They are used when surgery is contraindicated, as a bridge before surgery, or when disease recurs.

Metyrapone blocks 11-beta-hydroxylase, rapidly reducing cortisol. Typical doses range from 500 mg to 6 to 000 mg/day in divided doses. It can cause hirsutism in women because precursor androgens accumulate above the blocked step. A multinational retrospective study (N=195) found that metyrapone normalized UFC in 43% of patients at three months [13].

Ketoconazole (200 to 1 to 200 mg/day) inhibits multiple cytochrome P450 enzymes in the steroidogenesis pathway. Liver function monitoring is required given a risk of hepatotoxicity. Its use has declined with the availability of newer agents.

Osilodrostat (Isturisa) received FDA approval in March 2020 for adults with Cushing's disease for whom pituitary surgery is not an option or has not worked. The LINC 3 trial (N=137) showed normalization of UFC at 48 weeks in 86% of patients on stable doses (P<0.001) [14].

Mifepristone (Korlym, 300 to 1 to 200 mg/day) acts as a glucocorticoid receptor antagonist rather than reducing cortisol production. It is FDA-approved specifically for hyperglycemia in Cushing's syndrome. Because it blocks the receptor rather than the hormone, cortisol levels rise during treatment, making UFC an unreliable monitoring tool [15].

Adrenal Insufficiency: The Other Side of the Cortisol Axis

While Cushing's syndrome reflects too much cortisol, adrenal insufficiency reflects too little. Clinicians treating Cushing's patients must understand adrenal insufficiency because it is an expected consequence of curative treatment.

Primary adrenal insufficiency (Addison's disease) results from destruction of the adrenal cortex itself. Autoimmune adrenalitis causes 70 to 90% of cases in developed countries [3]. The adrenal glands cannot produce cortisol or aldosterone, leading to hyponatremia, hyperkalemia, hypotension, and hyperpigmentation (from compensatory ACTH elevation stimulating melanocortin receptors in skin). Fludrocortisone 0.05 to 0.2 mg once daily replaces aldosterone in addition to hydrocortisone.

Secondary adrenal insufficiency occurs when ACTH secretion from the pituitary is insufficient, whether from a pituitary tumor, pituitary surgery, radiation, or, most commonly, from prolonged exogenous glucocorticoid use. Aldosterone secretion is usually preserved because it is primarily regulated by the renin-angiotensin system rather than ACTH, so fludrocortisone is not routinely needed [16].

Adrenal crisis is a life-threatening emergency presenting with severe hypotension, vomiting, abdominal pain, and altered consciousness in the context of known or suspected adrenal insufficiency. Any patient with known adrenal insufficiency who develops an intercurrent illness, surgery, or significant physical stress needs immediate dose escalation. The standard "sick day rule" for adults is:

• Minor illness (fever, nausea): Double or triple the usual hydrocortisone dose for the duration of illness.
• Vomiting or unconsciousness: Administer hydrocortisone 100 mg intramuscularly or intravenously immediately and call emergency services.

A 2016 European survey of 1,245 adrenal insufficiency patients found an adrenal crisis rate of 8.3 per 100 patient-years, with 42% of crises triggered by gastrointestinal illness [17]. Every patient on long-term glucocorticoid therapy who is at risk for adrenal insufficiency should carry a steroid emergency card and a pre-filled hydrocortisone injection kit.

Distinguishing Cushing's Syndrome from Addison's Disease at a Glance

These two conditions sit at opposite ends of the cortisol spectrum and are sometimes confused by patients reading about adrenal disorders.

| Feature | Cushing's Syndrome | Addison's Disease | |---|---|---| | Cortisol level | High | Low | | ACTH (in primary forms) | High (if pituitary driven) or low (if adrenal driven) | High (primary) | | Skin changes | Thin, bruising, striae | Hyperpigmentation | | Blood pressure | High | Low | | Sodium | Normal to high | Low | | Potassium | Low (especially ectopic) | High | | Weight | Gain (central) | Loss | | Blood glucose | High | Low or normal |

Long-Term Outcomes and Mortality

Untreated or undertreated Cushing's syndrome carries a standardized mortality ratio of approximately 3.8 to 5.0 compared to the general population, driven primarily by cardiovascular disease, infection, and thromboembolic events [18]. Even after biochemical remission, cardiovascular risk factors may persist for years. A study of 80 patients in remission for a mean of 11 years found that 28% still had impaired glucose tolerance and 46% retained visceral adiposity [19].

Bone loss from prior hypercortisolism may not fully recover. Vertebral fracture screening with lateral spine X-ray or DXA with vertebral fracture assessment is appropriate at diagnosis and after remission. Bisphosphonate therapy may be indicated if T-score remains below minus 2.5 after cortisol normalization.

Psychiatric symptoms, particularly depression and cognitive dysfunction, improve substantially after treatment but may take 12 to 24 months to resolve fully. Quality of life in patients cured of Cushing's disease often remains below age-matched norms for up to three years post-surgery [20].

When to See a Specialist

A referral to an endocrinologist is appropriate when:

• Two or more classic features of cortisol excess are present without a clear exogenous source.
• An adrenal incidentaloma greater than 1 cm is found on imaging.
• A pituitary mass is identified on any imaging study.
• Screening tests show UFC greater than twice the upper reference limit on two separate collections.
• A patient on long-term systemic glucocorticoids shows signs of iatrogenic Cushing's syndrome and dose reduction is being planned.

Pituitary surgery for Cushing's disease should only be performed at centers completing at least 20 transsphenoidal procedures per year, where remission rates are meaningfully higher than at low-volume centers [10].