Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (845 page)

   The increment in plasma ACTH is the same in the morning and evening; however, the peak value is greater in the morning in normal subjects when the basal plasma ACTH concentration is higher. In contrast, the peak serum cortisol value is similar at both times of day, but the increment is smaller in the morning when the basal value is higher. In patients with Cushing syndrome, in whom the normal circadian rhythm in ACTH secretion is absent, the CRH test can be performed at any time of day with similar results.

   The response to CRH depends on the cause of the hypoadrenalism.

   Patients with primary pituitary ACTH deficiency (secondary adrenal insufficiency) have decreased plasma ACTH and serum cortisol responses to CRH.

   Patients with hypothalamic disease (i.e., CRH deficiency) usually have exaggerated and prolonged plasma ACTH responses; the plasma cortisol responses are subnormal.

   The CRH stimulation test is more reliable than the ACTH stimulation test in detecting pituitary–adrenal suppression in preterm infants whose mothers received a short course of dexamethasone before delivery to hasten fetal lung development.

CORTISOL FREE URINE, 24 HOURS

   Definition

   Cortisol free urine, 24-hour, or urinary free cortisol provides a direct and reliable practical index of cortisol secretion. It is an integrated measure of serum free cortisol level that is not affected by body weight.

   
Normal range:

   Males: <60 μg/day; <32 μg/g creatinine

   Females: <45 μg/day; <45 μg/g creatinine

   Use

   Aids in the diagnosis of hypercortisolism caused by Cushing syndrome

   Adrenal insufficiency (limited usefulness)