Why do low cortisol and low adrenocorticotropic hormone (ACTH) cause adrenal insufficiency?

Why Low Cortisol and Low ACTH Cause Adrenal Insufficiency

Low cortisol with low ACTH indicates secondary (or tertiary) adrenal insufficiency, where the problem originates in the pituitary gland (ACTH deficiency) or hypothalamus (CRH deficiency), not in the adrenal glands themselves. This represents a failure of the regulatory centers that normally stimulate cortisol production, rather than a failure of the hormone-producing adrenal glands 1.

Understanding the Mechanism

The hypothalamic-pituitary-adrenal (HPA) axis functions as a tightly controlled feedback system:

• Normal physiology: The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the pituitary to secrete ACTH, which in turn stimulates the adrenal glands to produce cortisol 2
• When ACTH is deficient: The adrenal glands receive insufficient stimulation and gradually atrophy, losing their capacity to produce adequate cortisol even though they are structurally capable of doing so 1

The key distinction: In secondary adrenal insufficiency (low ACTH + low cortisol), the adrenal glands themselves are intact but unstimulated. Over time, chronic lack of ACTH stimulation causes the adrenal cortex to atrophy, resulting in inadequate cortisol production 3, 4.

Distinguishing Primary from Secondary Adrenal Insufficiency

The ACTH-cortisol relationship is diagnostic:

• Primary adrenal insufficiency: High ACTH (>2-3× upper limit of normal) with low cortisol—the pituitary is desperately trying to stimulate failed adrenal glands 5, 6
• Secondary adrenal insufficiency: Low or inappropriately normal ACTH with low cortisol—the pituitary is not sending adequate signals 5, 6

Critical clinical difference: Patients with secondary adrenal insufficiency continue to produce aldosterone (controlled by the renin-angiotensin system, not ACTH), so they typically do not develop hyperkalemia or severe salt craving seen in primary disease 1. However, they remain at significant risk for life-threatening adrenal crisis 1.

Common Causes of Low ACTH Leading to Adrenal Insufficiency

The most frequent etiologies include:

• Exogenous glucocorticoid therapy: This is the most common cause of secondary adrenal insufficiency, affecting approximately 7 in 1,000 people on long-term oral corticosteroids—100 times more common than intrinsic adrenal disease 1
• Pituitary tumors or surgery: Mass effects or surgical damage to ACTH-producing cells 2, 1
• Lymphocytic hypophysitis: Often autoimmune, increasingly recognized with immune checkpoint inhibitor therapy 2, 5
• Hypothalamic lesions: Trauma, tumors, or infiltrative diseases affecting CRH production 4

Important caveat: Glucocorticoid suppression can occur with any route of administration—oral, inhaled, topical, intranasal, or intra-articular—particularly with prednisolone ≥5 mg daily (or hydrocortisone equivalent 10-15 mg/m² in children) for ≥1 month 1.

Clinical Consequences and Mortality Risk

The mortality implications are substantial:

• Increased all-cause mortality: Risk ratio of 2.19 for men and 2.86 for women with adrenal insufficiency, primarily from cardiovascular, infectious, and malignant diseases 1
• Adrenal crisis incidence: 6-8 episodes per 100 patient-years, with 8.3 per 100 replacement-years in prospective studies 1
• Deaths during crisis: Documented mortality during adrenal crisis, with highest risk in the first 3-6 months after cessation of long-term glucocorticoids 7

The fundamental problem: Without adequate cortisol, patients cannot mount appropriate stress responses to illness, surgery, or trauma, leading to cardiovascular collapse, severe hypoglycemia, and potentially fatal outcomes 1, 8.

Diagnostic Approach

When low cortisol is identified:

1. Measure morning ACTH simultaneously with 8 AM cortisol 9, 8
2. If ACTH is low with low cortisol (<3 μg/dL): Diagnosis of secondary adrenal insufficiency is established 5
3. If results are indeterminate (cortisol 3-15 μg/dL): Perform ACTH stimulation test, though this may give false-normal results in secondary AI 9
4. For definitive diagnosis of secondary AI: Insulin tolerance test or metyrapone test may be required 9
5. Evaluate other pituitary hormones: TSH, free T4, LH, FSH, testosterone/estrogen to assess for hypopituitarism 5, 6
6. Obtain pituitary imaging: MRI with sellar cuts if multiple hormonal deficiencies or new headaches/vision changes 5, 6

Critical pitfall: The standard ACTH stimulation test can produce false-normal results in early or partial secondary adrenal insufficiency because the adrenal glands may still respond to supraphysiologic ACTH doses despite inadequate endogenous stimulation 9, 3.

Why This Matters Clinically

Patients with low ACTH and low cortisol require lifelong glucocorticoid replacement to prevent adrenal crisis, but unlike primary adrenal insufficiency, they typically do not need mineralocorticoid (fludrocortisone) replacement since aldosterone production remains intact 1, 5, 6.

The condition is often underrecognized because symptoms are non-specific (fatigue, weakness, nausea, weight loss) and develop gradually 8, 10. However, during physiologic stress—infection, surgery, trauma—the inability to increase cortisol production can rapidly progress to life-threatening adrenal crisis characterized by hypotension, shock, and altered mental status 1.