ACTH stimulation test

Introduction

The anterior pituitary is often referred to as the “master gland” because, together with the hypothalamus, it orchestrates the complex regulatory functions of multiple other endocrine glands. The anterior pituitary gland produces six major hormones: (1) prolactin (PRL), (2) growth hormone (GH), (3) adrenocorticotropin hormone (ACTH), (4) luteinizing hormone (LH), (5) follicle-stimulating hormone (FSH), and (6) thyroid-stimulating hormone (TSH)

ACTH secretion is pulsatile and exhibits a characteristic circadian rhythm, peaking at 6 A.M. and reaching a nadir about midnight. Adrenal glucocorticoid secretion, which is driven by ACTH, follows a parallel diurnal pattern.

ACTION

The major function of the HPA axis is to maintain metabolic homeostasis and to mediate the neuroendocrine stress response. ACTH induces cortical steroidogenesis by maintaining adrenal cell proliferation and function. The receptor for ACTH, designated melanocortin- 2 receptor, is a GPCR that induces steroidogenesis by stimulating a cascade of steroidogenic enzymes

LABORATORY INVESTIGATION

Biochemical diagnosis of pituitary insufficiency is made by demonstrating low levels of trophic hormones in the setting of low target hormone levels. The diagnosis of adrenalInsufficiency may be established by means of an ACTH stimulation test.

Test

This is a screening test (the so-called rapid ACTH stimulation test) which involvesthe administration of 25 units (0.25 mg) of cosyntropin intravenously or intramuscularly and measurement of plasma cortisol levels before administration and 30 and 60 min after administration, the test can be performed at any time of the day. The most clear-cut criterion for a normal response is a stimulated cortisol level of >500 nmol/L (>18 µg/dL), and the minimal stimulated normal increment of cortisol is >200 nmol/L (>7 µg/dL) above baseline. Severely ill patients with elevated basal cortisol levels may show no further increases following acute ACTH administration.