HPA axis tests
Diagnostic tests assessing the HPA axis make use of the fact that it is regulated by negative feedback .
Glucocorticoid excess is diagnosed by employing a dexamethasone suppression test . Dexamethasone, a potent glucocorticoid, suppresses CRH/ACTH and, therefore, endogenous cortisol. If cortisol production is autonomous (e.g., adrenal nodule), ACTH is already suppressed and dexamethasone has little additional effect. If cortisol production is driven by an ACTH-producing pituitary adenoma, dexamethasone suppression is ineffective at low doses but usually induces suppression at high doses. If cortisol production is driven by an ectopic source of ACTH, the tumors are usually resistant to dexamethasone suppression. Thus, the dexamethasone suppression test is useful to establish the diagnosis of Cushing’s syndrome and to assist with the differential diagnosis of cortisol excess.
Conversely, to assess glucocorticoid deficiency , ACTH stimulation of cortisol production is used. The ACTH peptide contains 39 amino acids but the first 24 are sufficient to elicit a physiologic response. The standard ACTH stimulation test involves administration of cosyntropin (ACTH 1-24), 0.25 mg IM or IV, and collection of blood samples at 0, 30, and 60 minutes for cortisol. A normal response is defined as a cortisol level >20 ug/dL or an increment of >10 ug/dL over baseline. A low-dose (1 ug cosyntropin IV) version of this test has been advocated to avoid overstimulation of the adrenal gland.
Alternatively, an insulin tolerance test (ITT) can be used to assess adrenal insufficiency . It involves injection of insulin to induce hypoglycemia, which represents a strong stress signal that triggers hypothalamic CRH release and activation of the entire HPA axis. The ITT involves administration of regular insulin 0.1 U/kg IV (dose should be lower if hypopituitarism is likely) and collection of blood samples at 0, 30, 60, and 120 minutes for glucose, cortisol, and growth hormone (GH), if also assessing the GH axis. Oral or IV glucose is administered after the patient has achieved symptomatic hypoglycemia (usually glucose <40 mg/dL). A normal response is defined as a cortisol >20 ug/dL and GH >5.1 ug/L. The ITT requires careful clinical monitoring and sequential measurements of glucose. It is contraindicated in patients with coronary disease, cerebrovascular disease, or seizure disorders, which has made the short cosyntropin test the commonly accepted first-line test.
Mineralocorticoid production is controlled by the RAA regulatory cycle, which is initiated by the release of renin from the juxtaglomerular cells in the kidney, resulting in cleavage of angiotensinogen to angiotensin I in the liver . Angiotensin-converting enzyme (ACE) cleaves angiotensin I to angiotensin II, which binds and activates the angiotensin II receptor type 1 (AT1 receptor), resulting in increased aldosterone production and vasoconstriction. Aldosterone enhances sodium retention and potassium excretion, and increases the arterial perfusion pressure, which in turn regulates renin release. Because mineralocorticoid synthesis is primarily under the control of the RAA system, hypothalamic-pituitary damage does not significantly impact the capacity of the adrenal to synthesize aldosterone.
Similar to the HPA axis, the assessment of the RAA system can be used for diagnostic purposes. If mineralocorticoid excess is present, there is a counter-regulatory downregulation of plasma renin . Conversely, in mineralocorticoid deficiency, plasma renin is markedly increased. Physiologically, oral or IV sodium loading results in suppression of aldosterone, a response that is attenuated or absent in patients with autonomous mineralocorticoid excess.