Is the cause of this patient's condition adrenal or pituitary in origin?

This Patient Has Pituitary (Secondary) Adrenal Insufficiency

The dexamethasone suppression test results definitively indicate a pituitary origin: the morning cortisol of 3.9 with suppressed ACTH of 8 demonstrates appropriate suppression of the hypothalamic-pituitary-adrenal axis, ruling out autonomous cortisol production from either source. 1

Key Diagnostic Reasoning

Why This is NOT Adrenal Disease

• Normal imaging excludes adrenal adenoma: The CT adrenal protocol showed no adenoma, making primary adrenal hypercortisolism (Cushing's syndrome) extremely unlikely 2
• Appropriate cortisol suppression: The morning cortisol of 3.9 after dexamethasone administration demonstrates that the adrenal glands are responding normally to negative feedback 1
• Low ACTH with low cortisol: This pattern (ACTH 8 with morning cortisol 3.9) is pathognomonic for secondary (central) adrenal insufficiency, not primary adrenal disease 1

Why This is Pituitary-Related

• The ACTH-cortisol relationship is diagnostic: Low ACTH (8) paired with inappropriately low cortisol indicates the pituitary is not producing adequate ACTH to stimulate the adrenal glands 1
• Elevated TSH (5.2) supports central dysfunction: This TSH level of 5.2 is consistent with lack of cortisol's inhibitory effect on TSH production, a recognized finding in adrenal insufficiency 1
• Normal pituitary MRI does not exclude dysfunction: The absence of a visible adenoma does not rule out pituitary insufficiency or subtle dysfunction 2

Clinical Context Integration

The Weight Gain and Fatigue Pattern

• These symptoms reflect hormone deficiency, not excess: The patient's presentation is consistent with adrenal insufficiency (fatigue) and possible hypothyroidism (weight gain, fatigue), not Cushing's syndrome 1
• The dexamethasone suppression test was appropriate: This test confirmed there is NO autonomous cortisol production—the system suppressed normally 1

Additional Endocrine Considerations

• Screen for other autoimmune conditions: Given the TSH of 5.2, obtain thyroid antibodies (TPO-Ab) and free T4 to evaluate for concurrent autoimmune thyroid disease 1
• Monitor for progression: Annual screening should include TSH, free T4, glucose, HbA1c, and complete blood count 1

Critical Management Points

Immediate Actions Needed

• Do NOT treat for Cushing's syndrome: The suppression test rules this out completely 1
• Evaluate for secondary adrenal insufficiency: Perform an ACTH stimulation test (250 mcg cosyntropin) to assess adrenal reserve 1
• Assess thyroid function fully: Obtain free T4 and thyroid antibodies given the borderline elevated TSH 1

Common Pitfall to Avoid

• Do not confuse the clinical picture: Weight gain and fatigue can occur in both Cushing's syndrome AND adrenal insufficiency, but the biochemical testing clearly indicates insufficiency, not excess 1
• The pituitary microadenoma may be incidental: Not all pituitary lesions are functional; the biochemistry indicates hypofunction, not a hormone-secreting tumor 2, 3

If Adrenal Insufficiency is Confirmed

• Initiate glucocorticoid replacement: Hydrocortisone 15-25 mg daily in divided doses (typically 10 mg morning, 5 mg noon, 2.5-5 mg afternoon) 4
• Mineralocorticoid replacement is NOT needed: Secondary adrenal insufficiency preserves aldosterone production (controlled by renin-angiotensin system), unlike primary adrenal insufficiency 4
• Patient education is essential: Provide stress-dosing instructions and medical alert identification 1
• Start corticosteroids before thyroid hormone: If hypothyroidism is confirmed, always initiate glucocorticoid replacement several days before levothyroxine to prevent precipitating adrenal crisis 1