Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (843 page)

   The patient should be fasting 10–12 hours and should not take any corticosteroid, ACTH, or estrogen medications, if possible, for at least 48 hours prior to collection of specimen. An
AM
specimen is preferred. This test is rarely used.

   A rapid increase in circulating levels of CRH occurs at the onset of parturition.
1

   Plasma CRH concentrations do not correlate with plasma ACTH or serum cortisol concentrations or with altered hypothalamic–pituitary–adrenal axis function (e.g., in primary adrenal insufficiency or Cushing syndrome, or during insulin-induced hypoglycemia or metyrapone administration). Some investigators have reported a correlation between plasma CRH and plasma ACTH or serum cortisol in pregnancy, but others have not.

   The contribution of hypothalamic CRH to peripheral plasma CRH concentrations is small; most of the plasma CRH presumably comes from nonhypothalamic sources.

   However, under certain circumstances, such as insulin-induced hypoglycemia or during major surgery, small increments in plasma CRH concentrations may reflect hypothalamic CRH release.

CORTICOTROPIN-RELEASING HORMONE (CRH) STIMULATION TEST

   Definition

   CRH is a 41-amino-acid peptide, secreted by the paraventricular nucleus of the hypothalamus in response to stress. It acts on the anterior lobes of the pituitary to release ACTH. There is considerable sequence homology of CRH among species; as a result, both ovine and human CRH can be used in testing. Also known as: CRH after low-dose dexamethasone test.

   
Normal range:

   CRH stimulation test:

   Most patients with Cushing disease respond with ACTH and cortisol increases within 45 minutes after CRH. However, the criteria for interpretation have varied at different centers.

   Basal plasma ACTH concentrations increase 35–9,005 (mean 400%) in normal subjects and reach a peak of 10–120 pg/mL, 10–30 minutes after CRH injection; serum cortisol concentrations increase 20–600% (mean 250%) to 13–36 μg/dL (mean 25 μg/dL), reaching a peak 30–60 minutes after CRH injection.

   CRH after low-dose dexamethasone test: