Serum cortisol testing for suspected adrenal insufficiency
Case
A 45-year-old female with moderate persistent asthma is admitted for right lower extremity cellulitis. She has hyponatremia with a sodium of 129 mEq/L and reports a history of longstanding fatigue and lightheadedness on standing. An early morning serum cortisol was 10 mcg/dL, normal per the reference range for the laboratory. Has adrenal insufficiency been excluded in this patient?
Overview
Adrenal insufficiency (AI) is a clinical syndrome characterized by a deficiency of cortisol. Presentation may range from nonspecific symptoms such as fatigue, weight loss, and gastrointestinal concerns to a fulminant adrenal crisis with severe weakness and hypotension (Table 1). The diagnosis of AI is commonly delayed, negatively impacting patients’ quality of life and risking dangerous complications.1,2
AI can occur due to diseases of the adrenal glands themselves (primary) or impairment of adrenocorticotropin (ACTH) secretion from the pituitary (secondary) or corticotropin-releasing hormone (CRH) secretion from the hypothalamus (tertiary). In the hospital setting, causes of primary AI may include autoimmune disease, infection, metastatic disease, hemorrhage, and adverse medication effects. Secondary and tertiary AI would be of particular concern for patients with traumatic brain injuries or pituitary surgery, but also are seen commonly as a result of adverse medication effects in the hospitalized patient, notably opioids and corticosteroids through suppression the hypothalamic-pituitary-adrenal (HPA) axis and immune checkpoint inhibitors via autoimmune hypophysitis.
Testing for AI in the hospitalized patient presents a host of challenges. Among these are the variability in presentation of different types of AI, high rates of exogenous corticosteroid use, the impact of critical illness on the HPA axis, medical illness altering protein binding of serum cortisol, interfering medications, the variation in assays used by laboratories, and the logistical challenges of obtaining appropriately timed phlebotomy.2,3
Cortisol testing
An intact HPA axis results in ACTH-dependent cortisol release from the adrenal glands. Cortisol secretion exhibits circadian rhythm, with the highest levels in the early morning (6 a.m. to 8 a.m.) and the lowest at night (12 a.m.). It also is pulsatile, which may explain the range of “normal” morning serum cortisol observed in a study of healthy volunteers.3 Note that serum cortisol is equivalent to plasma cortisol in current immunoassays, and will henceforth be called “cortisol” in this paper.3
There are instances when morning cortisol may strongly suggest a diagnosis of AI on its own. A meta-analysis found that morning cortisol of < 5 mcg/dL predicts AI and morning cortisol of > 13 mcg/dL ruled out AI.4 The Endocrine Society of America favors dynamic assessment of adrenal function for most patients.2
Historically, the gold standard for assessing dynamic adrenal function has been the insulin tolerance test (ITT), whereby cortisol is measured after inducing hypoglycemia to a blood glucose < 35 mg/dL. ITT is logistically difficult and poses some risk to the patient. The corticotropin (or cosyntropin) stimulation test (CST), in which a supraphysiologic dose of a synthetic ACTH analog is administered parenterally to a patient and resultant cortisol levels are measured, has been validated against the ITT and is generally preferred.5 CST is used to diagnose primary AI as well as chronic secondary and tertiary AI, given that longstanding lack of ACTH stimulation causes atrophy of the adrenal glands. The sensitivity for secondary and tertiary AI is likely lower than primary AI especially in acute onset of disease.6,7
In performance of the CST a baseline cortisol and ACTH are obtained, with subsequent cortisol testing at 30 and/or 60 minutes after administration of the ACTH analog (Figure 1). Currently, there is no consensus for which time point is preferred, but the 30-minute test is more sensitive for AI and the 60-minute test is more specific.2,7,8
CST is typically performed using a “standard high dose” of 250 mcg of the ACTH analog. There has been interest in the use of a “low-dose” 1 mcg test, which is closer to normal physiologic stimulation of the adrenal glands and may have better sensitivity for early secondary or partial AI. However, the 250-mcg dose is easier to prepare and has fewer technical pitfalls in administration as well as a lower risk for false positive testing. At this point the data do not compellingly favor the use of low-dose CST testing in general practice.2,3,7
