Measurement of ACTH, CRH, and other hypothalamic and pituitary peptides

INTRODUCTION — Measurements of plasma corticotropin (ACTH) are extremely useful in the diagnosis of both Cushing's syndrome and adrenal insufficiency. In contrast, corticotropin-releasing hormone (CRH) is rarely measured in peripheral plasma or serum because the concentrations are much lower than and do not correlate well with those in the hypothalamic-hypophysial portal plasma. This is because of contribution from nonhypothalamic sources (eg, the gastrointestinal tract). The measurement of ACTH, CRH, and other related hypothalamic and pituitary peptides will be reviewed here. The approaches to the diagnosis of Cushing's syndrome and adrenal insufficiency are discussed separately. (See "Establishing the diagnosis of Cushing syndrome" and "Determining the etiology of adrenal insufficiency in adults".)

ACTH — The current commercially available two-site immunometric assays have an analytical sensitivity between 0.6 and 9 pg/mL (0.12 to 19.8 pmol/L) [1]. However, performance differs between assays and should be considered when interpreting patient results [2-4]. Furthermore, the most common platform immunoassay in use in the United States gives significantly different results in the critical range of 9 to 118 pg/mL (2 to 26.2 pmol/L) compared with a gold standard immunometric assay [5].

Normal values — Plasma corticotropin (ACTH) concentrations are usually between 10 and 60 pg/mL (2.2 and 13.3 pmol/L) at 8 AM. The values decrease during the waking hours and are usually less than 20 pg/mL (4.5 pmol/L) at 4 PM and less than 10 pg/mL (2.2 pmol/L), usually less than 5 pg/mL (1.1 pmol/L), within one hour after the usual time of falling sleep. This circadian rhythm in plasma ACTH concentrations is the cause of the parallel changes in cortisol secretion by the adrenal glands and the resulting rhythm in serum cortisol concentrations.

Interpretation — The time of day at which the sample is taken is an important determinant of the utility of the measurement of plasma ACTH, which is often best interpreted with a simultaneous measurement of serum cortisol.

In evaluating patients with adrenal insufficiency, measurements should be made in the morning, if possible, when plasma ACTH and serum cortisol concentrations are usually at their highest value [6] (see "Determining the etiology of adrenal insufficiency in adults"). However, in urgent or emergent evaluation of patients with potential adrenal insufficiency (eg, in the intensive care unit or emergency department), blood samples should be drawn at any time before administration of glucocorticoid therapy.

●Morning (8 AM [range 6 to 10 AM]) plasma ACTH is high and serum cortisol is low in primary adrenal insufficiency. Elevated plasma ACTH with a concurrent low serum cortisol at any time can indicate primary adrenal insufficiency.

●Both plasma ACTH and serum cortisol values are at the low end of the normal range or undetectable in secondary adrenal insufficiency (ie, hypopituitarism or isolated ACTH deficiency). This is also true in tertiary adrenal insufficiency (hypothalamic corticotropin-releasing hormone [CRH] deficiency), although this is rare and difficult to establish clinically. These values are inappropriately low for the degree of hypocortisolism.

●In congenital adrenal hyperplasia, early morning plasma ACTH values tend to be high, whereas serum cortisol may be normal or low, depending upon the severity of the enzyme deficiency.

Late evening plasma ACTH and serum cortisol values are of little value in diagnosing primary adrenal insufficiency or congenital adrenal hyperplasia, although evening plasma ACTH concentrations tend to be somewhat high in these disorders.

In the different forms of Cushing's syndrome, plasma ACTH and serum cortisol concentrations are most helpful when measured in the late evening, the time at which the concentrations are usually near their nadir in normal persons. These samples must be taken without stress, and this is impractical in most clinical circumstances. Whereas morning values may be within the normal range, there is loss of the normal circadian rhythm in secretion in Cushing's syndrome and the evening values are usually higher than normal. However, for the diagnostic purpose of distinguishing between ACTH-dependent tumors and primary adrenal causes of Cushing's syndrome, ACTH can be measured at any time [7]. (See "Establishing the diagnosis of Cushing syndrome".)

●In patients with Cushing's syndrome due to adrenocortical tumors, bilateral micronodular dysplasia, or other primary adrenocortical disorders (ie, ACTH-independent Cushing's syndrome), plasma ACTH concentrations are low, often undetectable, but serum cortisol concentrations are high. (See "Clinical presentation and evaluation of adrenocortical tumors" and "Cushing's syndrome due to primary bilateral macronodular adrenal hyperplasia" and "Cushing syndrome due to primary pigmented nodular adrenocortical disease".)

●In patients with Cushing's disease or ectopic ACTH syndrome (ie, ACTH-dependent Cushing's syndrome), the late evening plasma ACTH and serum cortisol concentrations are both high. The values for both tend to be higher in the ectopic ACTH syndrome than in Cushing's disease, but there is considerable overlap (see "Establishing the cause of Cushing syndrome", section on 'ACTH-dependent CS (ACTH >20 pg/mL)'). The morning plasma ACTH levels are often normal, but can be inappropriately elevated for the degree of hypercortisolism.

Pitfalls in measurement — Several factors may influence plasma ACTH assay results:

●ACTH may be unstable in blood at room temperature, is cleaved by enzymes in blood cells and platelets, and adheres to glass and some plastic surfaces [8]. Therefore, how blood is collected and plasma is prepared and stored may markedly affect the measured ACTH concentration; the severity of this problem varies with the antibodies used in the assays [9]. In general, it is recommended that ACTH blood samples be drawn into plastic tubes with EDTA and kept on ice until centrifugation, separation, and freezing of the plasma sample.

ACTH secretion in primary adrenal insufficiency, congenital adrenal hyperplasia, and ACTH-dependent Cushing's syndrome is episodic. Because the plasma disappearance half-time is measured in minutes, measurement of more than two samples may be useful, particularly if the initial result is equivocal. This can be done by obtaining samples at the same time on multiple days or by obtaining samples 30 to 60 minutes apart on the same day.

●Normal subjects and patients with adrenal disorders may respond rapidly to stress with increased ACTH secretion. For this reason, blood samples should be obtained with indwelling needles or catheters. Measurements in samples that require more than two or three minutes to obtain by venipuncture must be interpreted with caution, if high. For the same reason, it is best not to obtain post-sleep values the first night in the hospital before the patient is acclimated to a new environment.

●Patients with major depressive disorder, particularly older patients with severe melancholic depression, may have increased plasma ACTH concentrations [10]. (See "Establishing the diagnosis of Cushing syndrome", section on 'Exclude physiologic hypercortisolism'.)

●One must be certain that the patient is not taking or has not recently taken glucocorticoids, which can acutely or chronically suppress hypothalamic-pituitary-adrenal function.

●Two-site "sandwich" immunoassays for ACTH use two different monoclonal or affinity-purified polyclonal antibodies. They may not react with proopiomelanocortin (POMC) or forms intermediate between POMC and ACTH [1,11,12]. This specificity may be a disadvantage in diagnosing the ectopic ACTH syndrome, in which a large percentage of circulating immunoreactive ACTH may represent incompletely processed or unprocessed POMC (see 'Other POMC-derived peptides' below). POMC concentrations can be reliably measured only with a two-site POMC immunoassay [13], although this measurement is not widely commercially available.

In addition, excessive concentrations of ACTH fragments, such as ACTH-(1-24), can compete for binding to one or the other of the two antibodies, preventing intact ACTH from coupling to both antibodies and causing factitiously low ACTH concentrations [14]. This is unlikely to be a problem in measuring endogenous ACTH.

CRH — Like many other peptide hormones, corticotropin-releasing hormone (CRH) is measured by radioimmunoassay or enzyme-linked immunoassay [15]. In humans, CRH is largely bound to a plasma CRH-binding protein [16,17] and must be extracted by immunoaffinity chromatography or methanol before assay [18,19].

Peripheral plasma CRH concentrations range from about 1 to 10 pg/mL (0.2 to 2.2 pmol/L), increasing progressively during pregnancy to as high as 2700 pg/mL (585 pmol/L) [18,20]. The increased CRH in peripheral plasma in pregnant women is presumably produced by the placenta [21]. Most of the CRH is bound to CRH-binding protein [22]. The net effect of the increase in CRH is that maternal pituitary corticotropin (ACTH) secretion, plasma ACTH and serum cortisol concentrations, and urinary and salivary cortisol excretion increase during pregnancy [23]. This effect is gradual, apparently because the concentration of the CRH-binding protein falls during late pregnancy, resulting in a greater concentration of free bioactive CRH [24].

Despite an early report to the contrary [25], peripheral plasma CRH concentrations do not correlate with plasma ACTH or serum cortisol concentrations or with altered hypothalamic-pituitary-adrenal axis function (eg, in primary adrenal insufficiency or Cushing's syndrome, or during insulin-induced hypoglycemia or metyrapone administration). Some investigators reported a correlation between plasma CRH and plasma ACTH or serum cortisol in pregnancy [26], but others have not [27,28].

Plasma CRH concentrations are also high in the ectopic CRH syndrome, which is the one clinical setting in which measuring peripheral plasma CRH is of value [29-31].

OTHER POMC-DERIVED PEPTIDES — There is currently no clinical indication for measuring non-corticotropin (ACTH) proopiomelanocortin (POMC) hormones. POMC is the ACTH precursor peptide. It is also the source of beta-lipotropin (beta-LPH). In addition, ACTH and beta-LPH are cleaved further to yield alpha- and beta-melanocyte-stimulating hormone (alpha-MSH and beta-MSH), beta-endorphin, and met-enkephalin.

There is disagreement about the degree to which POMC is normally processed in the anterior pituitary [32,33], and differential processing may take place in different subpopulations of anterior pituitary corticotrophs. Therefore, POMC-derived peptides may not necessarily be released in equimolar concentrations. Furthermore, they appear to be metabolized at different rates, so that one may persist longer than others in the circulation. They are, however, released in concert so that their concentrations change in parallel [34]. Thus, the peripheral plasma concentrations of these hormones provide a direct index of pituitary secretion.

Radioimmunoassays and enzyme-linked immunoassays have been developed for POMC itself and other POMC-derived peptides, including alpha- and beta-MSH, beta-LPH, beta-endorphin, the N-terminal peptide of POMC, and the joining peptide between the N-terminal peptide and ACTH [13,32,35-39]. These assays have not been validated for routine clinical use.

Some early studies suggested that measurement of POMC or non-ACTH POMC fragments might have value in the diagnosis of ectopic ACTH secretion [40], or as a marker of tumor aggressivity in pituitary and non-pituitary tumors [41]. [42]. However, measurement of non-ACTH POMC hormones is currently not clinically indicated.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Diagnosis and treatment of Cushing syndrome" and "Society guideline links: Adrenal insufficiency".)

SUMMARY — Measurements of plasma corticotropin (ACTH) are important for the diagnosis of both Cushing's syndrome and adrenal insufficiency. Corticotropin-releasing hormone (CRH) measurements are less useful clinically, but are sometimes used in the evaluation of ectopic CRH syndrome. (See 'CRH' above.)

When evaluating patients with adrenal insufficiency, measurements should be made in the morning if possible, when plasma ACTH and serum cortisol concentrations are usually at their highest value. (See 'ACTH' above and "Determining the etiology of adrenal insufficiency in adults".)

However, elevated plasma ACTH in the presence of low serum cortisol can be diagnostic for primary adrenal insufficiency at any time of day.

●In primary adrenal insufficiency, early morning (6 to 10 AM) plasma ACTH is high and serum cortisol is low in primary adrenal insufficiency.

●In secondary and tertiary adrenal insufficiency, plasma ACTH values are low or within the reference range (but inappropriately low for the degree of hypocortisolism). Serum cortisol is usually low or at the low end of the reference range.

In the different forms of Cushing's syndrome, plasma ACTH and serum cortisol concentrations can be measured at any time.

●In patients with Cushing's syndrome due to adrenocortical tumors, bilateral micronodular dysplasia, or other primary adrenocortical disorders (ie, ACTH-independent Cushing's syndrome), plasma ACTH concentrations are low, often undetectable, but serum cortisol concentrations are high. (See 'ACTH' above and "Clinical presentation and evaluation of adrenocortical tumors" and "Cushing's syndrome due to primary bilateral macronodular adrenal hyperplasia" and "Cushing syndrome due to primary pigmented nodular adrenocortical disease".)

●In patients with Cushing's disease or ectopic ACTH syndrome (ie, ACTH-dependent Cushing's syndrome), the plasma ACTH and late evening serum cortisol concentrations are both high. Plasma ACTH can be within the reference range in the morning, but is usually inappropriately elevated for the degree of hypercortisolism. The values for both tend to be higher in the ectopic ACTH syndrome than in Cushing's disease, but there is considerable overlap. (See "Establishing the cause of Cushing syndrome", section on 'ACTH-dependent CS (ACTH >20 pg/mL)'.)

●Measurement of CRH is not indicated for clinical purposes except in the setting of suspected ectopic CRH secretion in Cushing’s syndrome. (See 'CRH' above.)

●Measurement of non-ACTH proopiomelanocortin (POMC) hormones is not clinically indicated. (See 'Other POMC-derived peptides' above.)

ACKNOWLEDGEMENT — 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.