Adrenocortical Functions

ANATOMICALLY: HISTOLOGICALLY: The adrenal gland is situated on the anteriosuperior aspect of the kidney and receives its blood supply from the adrenal arteries. HISTOLOGICALLY: The adrenal gland consists of two distinct tissues of different embryological origin, the outer cortex and inner medulla.

The adrenal cortex comprises three zones based on cell type and function: Zona glomerulosa The outermost zone  aldosterone (the principal mineralocorticoid). Zona fasciculata The deeper layers of the cortex  glucocorticoids – mainly cortisol (95%) is the most important in man. Zona reticularis  androgen production .

Glucocorticoid secretion & function Cortisol is an insulin antagonist and has a weak mineralocorticoid action Glucocorticoids have widespread metabolic effects on carbohydrate, fat and protein metabolism. In the liver :  Gluconeogenesis  production of glucose  Amino acid uptake and degradation  Ketogenesis.

In the adipose tissue: Cortisol   Lipolysis. In the muscles: Cortisol  proteolysis and amino acid release.

Plasma [CBG] : In the circulation, glucocorticoids are mainly protein-bound (about 90%), chiefly to corticosteroid-binding globulin (CBG or transcortin).  in pregnancy and with estrogen treatment (e.g. oral contraceptives).  in hypoproteinemic states (e.g. nephrotic syndrome). The biologically active fraction of cortisol in plasma is the free (unbound) component .

Regulation of ACTH and cortisol secretion : ACTH is the main stimulus to cortisol secretion. 1. Negative feedback control: ACTH release from the anterior pituitary is stimulated by hypothalamic secretion of corticotrophin releasing hormone (CRH).  plasma [cortisol] or synthetic glucocorticoids supress secretion of CRH and ACTH. CRH   ACTH   [Cortisol]   ACTH & CRH   Cortisol

hypothalamic-pituitary-adrenal axis Negative Feed back

2. Stress (e.g. major surgery, emotional stress) Stress   CRH & ACTH   Cortisol 3. The diurnal rhythm of plasma cortisol: Highest Cortisol level in the morning ( 8 - 9 AM ). Lowest Cortisol level in the late afternoon and evening ( 8 - 9 PM ).

Aldosterone secretion: The principal physiological function of aldosterone is to conserve Na+, mainly by facilitating Na+ reabsorption and reciprocal K + or H+ secretion in the distal renal tubule. aldosterone is a major regulator of water and electrolyte balance, as well as blood pressure.

A. The renin - angiotensin system: It is the most important system controlling aldosterone secretion. I. Renin: a proteolytic enzyme produced by the juxtaglomerular cells of the afferent renal arteriole. released into the circulation in response to : a fall in circulating blood volume. renal perfusion pressure. loss of Na+.

The renin – angiotensin system The renin – angiotensin system. Renin is released from the renal Juxtaglomerular cells in response to hypotension , low blood volume or sodium depletion

2. Potassium ions:  plasma K+   Aldosterone release  exerts its effects on the distal renal tubule   K+ output in the urine. 3. ACTH: ACTH is relatively unimportant ( except in stress, CAH.

Androgen secretion : Cells of the zona reticularis provide androgens such as : Androstenedione Dehydroepiandrosterone (DHEA). Dehydroepiandrosterone sulphate. These hormones : leads to  enhance muscle mass. stimulate cell growth. aid in the development of the secondary sexual characteristics. The measurements of these androgens are important in the investigation of hirsuitism, verilisation and CAH

Cushing’s Syndrome An endocrine disorder caused by : Excessive levels of the endogenous corticosteroid hormone (cortisol). (relatively rare) Induced iatrogenically by treatment with exogenous corticosteroids for other medical conditions. (the most common form of Cushing's syndrome).

Cushing’s Syndrome … cont. Clinical features: Hypertension Muscle weakness Purple striae Excessive sweating Central obesity Glucose intolerance Psychological disturbances Hirsuitism

CAUSES OF ADRENOCORTICAL HYPERFUNCTION: ACTH - dependent : 1. Pituitary adenoma (Cushing's disease). 2. Ectopic production of ACTH by neoplasms. 3. Ectopic CRH or related peptides. 4. ACTH therapy. ACTH - independent : 1. Adrenal adenoma. 2. Adrenal carcinoma. 3. Glucocorticoid therapy.

Investigations of suspected adrenocortical hyperfunction A. Screening tests (out-patient): to assess the clinical diagnosis of adrenocortical hyperfunction. B. Confirmatory tests (in-patient): to confirm or exclude the provisional diagnosis C. Tests to determine the cause: to ascertain: (a) The site of the pathological lesion (adrenal cortex, pituitary or elsewhere?) (b) The nature of the pathological lesion.

A. Screening tests: Overnight suppression test: The screening tests serve to distinguish simple non- endocrine obesity from obesity due to Cushing's syndrome. Effective screening tests need to be sensitive but do not have to be specific, it includes: Low-dose dexamethasone suppression test : (outpatient procedure) Overnight suppression test: Procedure: 1 mg dexamethasone administered at 11-12 PM the night before attending the clinic . serum cortisol is measured at 8- 9 AM. Interpretation: Under normal circumstances, dexamethasone supresses the secretion of CRH, ACTH and cortisol levels. Dexamethasone   CRH   ACTH   cortisol. Cortisol < 50 nmol/L  exclude Cushing’s disease.

False abnormal results: Drugs that induce hepatic microsomal enzymes ( Phenobarbitone & phenytoin)   dexamethasone metabolism  dexamethasone effect (False negative results)

2. 24- hour urinary free cortisol: Cortisol is removed from plasma by the liver  metabolically inactive compounds  excreted in urine mainly as conjugated metabolites (e.g. glucuronides). A small amount of cortisol is excreted unchanged in the urine. Urinary cortisol excretion is related to the biologically active plasma [free cortisol] during the period of urine collection.

Interpretation of the results: In normal individuals: Urinary cortisol excretion is < 250 nmol/24 h. Cortisol / Creatinine ratio in an early morning specimen of urine is < 25 μmol cortisol / mol creatinine.

Interpretation of screening tests: False positive results may be due to: in depressed or extremely anxious patients in the presence of severe intercurrent illness in alcoholism (pseudo-Cushing's syndrome).

B. Confirmatory tests : (Inpatient) To rule out pseudoCushing's syndrome and determine the specific cause of the adrenocortical hyperfunction. Patients need to be hospitalized for at least 48 hours prior to stress-free venepuncture (e.g. in-dwelling catheter) to obtain meaningful results. 1. Diurnal rhythm of plasma cortisol: The morning plasma [cortisol] is normal, but the evening concentration is raised. Patients with Cushing’s syndrome lose the diurnal rhythm . It is also convenient to collect a series of 24-hour urine samples for urine free cortisol measurement.

2. Insulin hypoglycemia test: To test the integrity of the hypothalamic-pituitary-adrenal (HPA) axis. To distinguish true Cushing's syndrome from pseudoCushing’s syndrome. Contraindicated in: epilepsy or heart disease.

Insulin hypoglycemia test …. Contd Procedure: Insulin I.V. (0.15 U/kg) to lower blood glucose to 2.2 mmol/L or less . Samples for simultaneous measurement of glucose and cortisol are taken basally (before insulin) and at 30, 45, 60 and 90 min after intravenous insulin injection. Failure to achieve a glucose level of 2.2 mmol/L invalidates the test and should be repeated with insulin increment. Interpretation : Hypoglycemia  CRH  ACTH  cortisol

Interpretation of the results: Normally: Basal serum [cortisol] : 250- 300 nmol/L At 60 - 90 minutes : the level > 425 nmol/L with an increment above the basal level of at least 145 nmol/L . Patients with Cushing's syndrome: whatever the cause, show little or no increase in serum [cortisol] and do not respond normally to insulin-induced hypoglycemia. Basal serum [cortisol] is high than normal . At 60 - 90 minutes : no increase in serum [cortisol], with increment less than 145 nmol/L despite the production of an adequate degree of hypoglycemia.

Determining the cause of Cushing’s syndrome 1. Plasma [ACTH] : Plasma [ACTH] should be measured on blood specimens collected at 8-9 a.m. and 8-9 p.m. Functional adrenal tumor  confirmed by an abdominal CT scan to detect an adrenal mass Ectopic (non-endocrine) origin of ACTH

2. High-dose dexamethasone suppression test: Used to distinguish Cushing's disease from ectopic ACTH secretion 2 mg dexamethasone six-hourly for 48 hours to suppress cortisol secretion. Basal (pre-dexamethasone) serum cortisol or 24-hour urine free cortisol is compared with the same at the end of the 48-hour period. Suppression is defined as a fall to less than 50 % of basal value.

About 90 % of patients with Cushing's disease show suppression of cortisol output. In contrast, only 10% of patients with ectopic ACTH production (or with adrenal tumors) show suppression. An overnight suppression test using a single dose of 8 mg dexamethasone show similar diagnostic accuracy to the standard 48-hour high-dose test.

Ectopic ACTH & adrenal tumors  ACTH & cortisol above basal 3. CRH stimulation test: Measures the ACTH and cortisol levels basally and15.30.45. 60 .90 & 120 minutes after injection of 100 g CRH. Ectopic ACTH & adrenal tumors Normal Cushing's disease  ACTH & cortisol  ACTH & cortisol above basal No response In Cushing's disease: High-dose dexamethasone suppression test + the CRH test  100 % specificity and sensitivity.

Other chemical Investigations: Potassium: Hypokalemic alkalosis may be a prominent feature of ectopic ACTH production. Patients with Cushing's syndrome are often  treated with diuretics (for hypertension and edema), and this treatment   plasma [K+]. Pituitary function tests: may be abnormal in Cushing's syndrome due to the suppressive effect of cortisol on the hypothalamus and pituitary.

Other Confirmatory Investigations: CT scanning of the adrenal gland . MRI of the pituitary gland.

Adrenal Hyperfunction Summary of Biochemical Tests Cushing’s disease Adrenal tumor Ectopic ACTH secreting tumor S. cortisol  Dexamethasone Low dose test Not suppressed Urinary cortisol Diurnal rhythm Lost Insulin-induced hypoglycemia No response Plasma [ACTH] Normal or  Not detectable    Dexamethasone High dose test suppressed CRH test