INTRODUCTION — The adrenal cortex consists of three functional zones [1,2]:
●The zona glomerulosa, which secretes aldosterone and is primarily under the control of the renin-angiotensin system
●The zona fasciculata, which secretes cortisol and is regulated by the corticotropin-releasing hormone (CRH)-corticotropin (ACTH) system
●The zona reticularis, which secretes dehydroepiandrosterone (DHEA), DHEA sulfate, and smaller amounts of androstenedione, testosterone, and other 19-carbon steroids and is under the control of ACTH and perhaps other proopiomelanocortin (POMC)-derived peptides
Consequently, evaluation of adrenal function requires measurement of the relevant adrenal hormones and their metabolites and sometimes of the secretagogues (ACTH levels for cortisol or renin activity or concentration for aldosterone) that regulate their secretion.
This topic will review the basic principles that should be considered when using these tests to determine if a patient has some abnormality in adrenal hormone synthesis. The specific diagnostic approach to disorders such as Cushing's syndrome, adrenal insufficiency, and hyperaldosteronism are discussed separately. (See "Establishing the diagnosis of Cushing's syndrome" and "Diagnosis of adrenal insufficiency in adults" and "Diagnosis of primary aldosteronism".)
REFERENCE RANGES — The reference ranges referred to in the discussion of various assays are intended only as general guidelines. Each laboratory has its own assay procedures and normal or reference ranges. Reference ranges should, in general, be viewed with some skepticism with regard to how they were established, particularly if they deviate significantly from those listed in the discussions of the individual tests.
Salivary and circulating hormone levels may have a diurnal rhythm. For certain hormones such as cortisol, the variations can be quite large, with increases to very high levels a few hours before awakening, pulsatile decreasing values throughout the day, and nadir values in the evening and early part of the sleep period [3]. Time-specific normal ranges should be sought for these tests, and test strategies should be planned with this in mind. As an example, measurement of late evening, but not early morning, serum or salivary cortisol levels is useful for the diagnosis of Cushing's syndrome, whereas measurement of early morning, but not late evening, serum or salivary cortisol levels is useful for the diagnosis of adrenal insufficiency. Clinicians should verify whether their patients have unusual sleep hour patterns (eg, night shift work).
In addition, other tests that evaluate the function of systems that are regulated by nonhormonal factors may not have broadly applicable "normal" values. This problem is illustrated by the renin-angiotensin-aldosterone system. The activity of this system is regulated in part by the effective arterial blood volume, which is determined in normal subjects by sodium intake. Thus, interpretation of measurements of plasma renin activity and serum aldosterone requires knowledge of the supine or upright posture of the subject and the level of sodium intake, which, in the steady state, can be estimated from 24-hour urinary sodium excretion. The aldosterone/renin ratio is often required to interpret the integrity of this axis. (See "Assays of the renin-angiotensin-aldosterone system in adrenal disease".)
LABORATORY AND PATIENT ERRORS — A clinician is only as good as the reference laboratory. Because the symptoms and signs of adrenocortical disorders are often subtle and nonspecific, assessment of basal hormone secretion may not establish the diagnosis, and even the most reliable laboratory is capable of occasional error. Regrettably, some reference laboratories may provide erroneous results for one or more of the assays used for evaluation of the hypothalamic-pituitary-adrenal (HPA) axis. The assays most subject to error are the serum and salivary cortisol assays (especially at low values) and the plasma corticotropin (ACTH) values (especially in the low range values).
The choice of assay can affect the results. For example, urine cortisol can be measured commercially by liquid chromatography/mass spectrometry, based on its structure, or by an antibody-based assay. The structurally based assays have little cross-reactivity with other endogenous or exogenous glucocorticoids. By contrast, other steroids may cross-react in the antibody-based assays and contribute to the higher reference ranges for those assays. In addition to errors in the laboratory, patients can inadvertently contribute to inaccurate tests with incomplete collections or by omitting to report the use of interfering drugs or "nonprescription products or supplements." (See "Measurement of urinary excretion of endogenous and exogenous glucocorticoids" and "Measurement of ACTH, CRH, and other hypothalamic and pituitary peptides".)
Detecting inconsistencies in lab data — One cannot detect all laboratory errors, but there are some useful principles for evaluating the results. Most important is awareness that any result may be incorrect, particularly when it conflicts with otherwise consistent data.
As an example, if urinary cortisol is elevated, one would expect that the evening cortisol in serum or saliva would be elevated as well. With regard to these three assays, salivary or urinary cortisol is most likely to be correct and evening cortisol is more susceptible to errors. Conflicting results should be checked and repeated in the same or another laboratory. A diagnosis should not be made in the face of inconsistent laboratory data.
There is an additional potential error when applying these principles to urinary cortisol excretion in states of marked hypercortisolism. When the serum cortisol concentration exceeds the normal binding capacity of cortisol-binding globulin (CBG) of approximately 25 mcg/dL (690 nmol/L), there will be a proportionately greater rise in cortisol excretion since more cortisol in serum is unbound.
The above relationships between urinary and serum cortisol presumes that the CBG concentration is normal. Low CBG concentrations, as may be seen in genetic deficiency, critical illness, or nephrotic syndrome, result in a lower than expected serum cortisol value [4,5]. Conversely, high CBG concentrations, as seen with oral estrogen administration, increase serum cortisol concentrations [4]. In the latter case, a 24-hour urinary free and salivary cortisol better reflects adrenal function.
Patient errors — There are a few major sources of patient errors:
●Failure to take prescribed medications or to take them on the proper schedule
●Failure to inform the clinician of other medications (such as exogenous glucocorticoid-containing drugs, creams, or injections) that may interfere with the diagnostic tests, the hormonal assays, or both
●Failure to collect 24-hour urine specimens properly
●Failure to collect salivary cortisol properly, resulting in contamination of the specimen with cortisol-containing topical agents
The first two must be dealt with by diligence on the part of both clinician and patient, particularly when tests are increasingly being conducted on an ambulatory basis. Improper 24-hour urine collections can be minimized by explaining to the patient in detail how the samples should be collected, emphasizing how important they are to the diagnosis, and making the patient (either in or out of hospital) fully responsible for their accuracy. The patient should then be requested to repeat exactly how the urine is to be collected to be certain that the instructions are understood. It is helpful to have printed instructions to which the patient can refer (see "Patient education: Collection of a 24-hour urine specimen (Beyond the Basics)"). Additionally, the completeness of the collection may be assessed by measuring the volume and the creatinine. (See "Measurement of urinary excretion of endogenous and exogenous glucocorticoids".)
SUMMARY — Evaluation of adrenal function requires measurement of the relevant adrenal hormones, their metabolites, and often the secretagogues that regulate their secretion (mainly corticotropin [ACTH] and renin levels). Correct interpretation of results depends upon a number of factors including:
●Source and accuracy of normative data for any given assay
●Appropriate timing of sample collection (eg, cortisol secretion has a diurnal rhythm)
●Identifying laboratory error and patient errors in medication compliance and/or sample collection
Specific assays are discussed in detail separately. (See "Assays of the renin-angiotensin-aldosterone system in adrenal disease" and "Measurement of cortisol in serum and saliva".)
DISCLOSURE — The views expressed in this topic are those of the author(s) and do not reflect the official views or policy of the United States Government or its components.
