Speciality Immunoassays and Reagents

ACTH ELISA

ACTH ELISA				Catalog # ACTH.96
Format	Sandwich ELISA				View product insert
Size/Volume	96 wells
Species	human, mouse, rat				Request Quote
Sample Type	plasma
Sample Preparation	-
Sample Size	200 uL
Standard Curve Range	0 - 500 pg/mL
Sensitivity	0.46 pg/mL
Assay Length	4.5 hrs

 

Summary and explanation

ACTH (Adrenocorticotropic hormone) or corticotropin is a 39-amino acid peptide hormone (MW=4500) secreted by the pituitary to regulate the production of steroid hormones by the adrenal cortex. ACTH secretion from the anterior pituitary is controlled by both a classical negative feedback control mechanism and CNS-stress mediated control system. Various types of stress or pain perceived in higher levels of the brain modulate secretion of the hypothalamic neurosecretory hormone, corticotropin releasing hormone (CRH), a 41-amino acid peptide. CRH stimulates pituitary ACTH secretion. The second peptide that modulates ACTH secretion is vasopressin (AVP). AVP secretion is also stimulated by stress and acts synergistically with CRH to increase ACTH secretion in the pituitary portal circulation. ACTH increases the synthesis and release of all adrenal sterioids, aldosterone, cortisol and adrenal androgens. It is the principal modulator of cortisol, the most important glucocorticoid in man. As the cortisol level in blood increases, release of ACTH is inhibited directly at the pituitary level. Through this same mechanism, decreasing cortisol levels lead to elevated ACTH levels.

Biologically active ACTH results from enzymatic cleavage of a large precursor molecule, pro-opiomelanocortin (POMC). This molecule contains within its structure the amino acid sequences of ACTH, Pro-ACTH, -melanocyte stimulating hormone, lipotropin, as well as endorphin and the enkephalins. Because the reaction in immunoassays is determined by antigenic structure, not biological function, the usual ACTH RIA reacts with POMC, Pro-ACTH, ACTH and some fragments of the ACTH.

Clinical significance

Plasma ACTH assays are useful in the differential diagnosis of pituitary Cushing's disease, Addison's disease, autonomous ACTH producing pituitary tumors (e.g. Nelson's syndrome), hypopituitarism with ACTH deficiency and ectopic ACTH syndrome.

Cushing's syndrome is caused by the effects of excess glucocorticoid actions. All causes of Cushing's syndrome, with the exception of glucocorticoid medication, are associated with incresed 24 hour urinary cortisol. The most common cause of Cushing's syndrome is bilateral adrenal hyperplasia, due to pituitary ACTH hypersecretion (Cushing's disease) from a pituitary adenoma or corticotroph hyperplasia. Laboratory diagnosis of Cushing's disease is supported by the following: (1) suppression of plasma ACTH and cortisol concentrations, by high-dose (2.0 mg q 6h x 8) dexamethasone administration; (2) absence of ACTH and cortisol suppression with low-dose (0.5 mg q 6h x 8 or 1 mg given at 23:30 hour) dexamethasone; (3) larger than normal response to metyrapone (Metopirone ) stimulation and normal or elevated plasma ACTH levels.

When Cushing's syndrome is caused by primary adrenal abnormality (adenoma or carcinoma), the adrenal gland acts independently of ACTH and pituitary ACTH secretion is suppressed. Hence, there is no response to dexamethasone suppresion or metyrapone stimulation. This type of Cushing's syndrome is characterized by very low or undetectable levels of ACTH.

Therefore, measurement of plasma ACTH is helpful in differential diagnosis of pituitary Cushing's syndrome. In patients with adrenal tumors ACTH levels are low. High levels of ACTH are seen in patients with ectopic ACTH syndrome. Patients with bilateral adrenal hyperplasia will have ACTH levels inappropriately elevated for their degree of hypercortisolism, which should suppress ACTH. However, in most cases the ACTH concentration will be within the normal range.

Adrenocortical insufficiency or inadequate cortisol production can be due to destruction of the adrenal cortex or to abnormalities of the pituitary or hypothalamus which result in inadequate ACTH production of release. Primary adrenocortical insufficiency, Addison's disease, is characterized by markedly elevated plasma ACTH levels and adrenal unresponsiveness to stimulation with exogenous ACTH. Hypopituitarism with ACTH deficiency, which is secondary adrenocortical insufficiency, is characterized by low plasma ACTH and cortisol concentrations, and a subnormal, but usually distinct adrenal response to stimulation with synthetic ACTH (Cortrosyn®). If hypoglycemic stress or metyrapone stimulation is required for diagnosis, ACTH and cortisol responses are less than normal.

Aggressive and invasive ACTH producing pituitary tumors occurring before or following bilateral adrenalectomy for Cushing's disease (Nelson's syndrome) are characterized by the development of Addisonian pigmentation, often in an adrenalectomized patient who is taking adequate glucocorticoid replacement therapy. In these patients, plasma ACTH levels are markedly elevated and do not respond well to dexamethasone suppression.

Note: The second reading is designed to extend the analytical validity of the calibration curve to the value represented by the highest calibrator, which is approximately 500 pg/mL. Hence, patient samples with ACTH > 150 pg/mL can be quantified against a calibration curve consisting of the readings all the way up to the concentration equivalent to the highest calibrator using the 405 nm reading, away from the wavelength of maximum absorbance.In general, patient and control samples should be read using the 450 nm for ACTH concentrations up to 150 pg/mL. ACTH concentrations above 150 pg/mL should be interpolated using the 405 nm reading.

By using the final absorbance values obtained in the previous step, construct a calibration curve via cubic spline, 4 parameter logistics, or point-to-point interpolation to quantify the concentration of the ACTH.

 

Product inserts are for information use only.

MDB ACTH ELISA MSDS