The adrenal gland
August 24th, 2010 |Anatomy
Adrenal glands are small triangular organs located on top of the kidneys. The adrenal gland consists of a medulla (the center of the gland) surrounded by the cortex. Epinephrine (adrenaline) and norepinephrine (noradrenaline) are produced by the medulla. The adrenal cortex is the source of a number of steroid hormones and is arranged as 3 major layers or zones, organized as concentric shells:
- The zona glomerulosa produces mineralocorticoids (aldosterone as the principal one) and cannot produce cortisol or androgens (male sex hormones).
- The zona fasciculata is the thickest layer of the adrenal cortex, making up about 75% of the cortex. It produces glucocorticoids (cortisol) and androgens.
- The inner zona reticularis surrounds the medulla and it also produces cortisol and androgens.
The zona fasciculata and the zona reticularis are regulated by a hormone secreted by the pituitary gland, ACTH (Adreno Cortico Tropic Hormone). Excess or a deficiency of this hormone alters the structure and function of these zones: when ACTH is deficient they atrophy (degenerate); when ACTH is present in excess, hyperplasia and hypertrophy of these zones occur. ACTH in turn is regulated by the hypothalamus and central nervous system (CNS) via neurotransmitters and corticotropin-releasing hormone (CRH) and arginine vasopressine (AVP).
Biosynthesis of cortisol & adrenal androgens
Synthesis of cortisol and the androgens begins with cholesterol. Plasma lipoproteins are the major source of adrenal cholesterol, though synthesis within the adrenal gland from acetate also occurs. Low-density lipoprotein (LDL) accounts for about 80% of cholesterol delivered to the adrenal gland. Cholesterol is stored as cholesterol-esters in the lipid droplets within the adrenocortical cells.
The adrenal androgens DeHydroEpiAndrosterone (DHEA), its sulfate ester DHEA-S and androstenedione have minimal intrinsic androgenic activity but are converted in the tissues of the body to the more potent androgens testosterone and dihydrotestosterone (male sex hormones) and to oestrogens (female sex hormone). DHEA and DHEA-S are the most abundantly produced steroids by the adrenal cortex. DHEA and DHEA-S are readily interconvertible and will from now on be called DHEA(S).
Control of cortisol secretion
Cortisol secretion is closely regulated by ACTH, and the plasma cortisol levels parallel those of ACTH. There are 3 mechanisms of neuroendocrine control:
- Circadian (day and night) rhythm of ACTH.
- Stress responsiveness of the hypothalamic-pituitary adrenal (H-P-A) axis.
- Feedback inhibition by cortisol of ACTH secretion.
Plasma ACTH and cortisol are secreted within minutes following the onset of stress and these responses abolish circadian periodicity if the stress is prolonged.
Control of adrenal androgen secretion
DHEA(S) is the most abundantly produced steroid in men. Under normal circumstances, DHEA(S) is secreted synchronously with cortisol in response to CRH and ACTH. DHEA-S concentration demonstrates only limited day-night variation due to its long circulating half-life and lower clearance.
In the human fetus the so-called fetal zone of the adrenal cortex produces DHEA(S) in high quantities and cortisol production only starts at the end of pregnancy.
DHEA(S) displays a characteristic secretion pattern over a lifetime, with a surge during the prepubertal period (adrenarche, ~6 years), reaching a maximum at 25-35 years of age followed by a continuous decline to steadily low levels with advancing age (adrenopause).
Circulation of cortisol and adrenal androgens
Cortisol and adrenal androgens are secreted into the blood in an unbound state; however, these hormones bind to plasma proteins upon entering the circulation. Cortisol binds mainly to corticosteroid-binding globulin (CBG) and to a lesser extent to albumin, whereas the androgens bind mainly to albumin. Under basal conditions, about 10% of the circulating cortisol is free, approximately 75% is bound to CBG and the remainder to albumin.
Bound steroids are biologically inactive; the unbound or free fraction is active.
Biological effects of adrenal steroids
Adrenal steroids like cortisol were originally called glucocorticoids because of their influence on glucose metabolism. Cortisol maintains the plasma glucose levels during fasting and increases plasma glucose during stress by stimulating gluconeogenesis and inhibiting glucose uptake in muscle and adipose tissue. Glucocorticoid receptors are present in virtually all tissues and have a variety of other effects. Cortisol has an inhibitory effect on the immune system and the inflammatory response. By its effects on the central nervous system cortisol influences mood, appetite, sleep and memory.
Aldosterone maintains the right amount of sodium, potassium and water in the body.
Pathophysiology
Loss of more than 90% of both adrenal cortices results in the clinical manifestations of adrenocortical insufficiency. With gradual adrenocortical destruction, the initial phase is that of decreased adrenal reserve; ie, basal steroid secretion is normal, but secretion does not increase in response to stress. Thus, acute adrenal crisis can be precipitated by the stresses of surgery, trauma or infection. With further loss of cortical tissue, even basal secretion of mineralocorticoids and glucocorticoids becomes deficient, leading to manifestations of chronic adrenocortical insufficiency.
With decreased cortisol secretion, plasma levels of ACTH are increased because of decreased negative feedback inhibition of their secretion. In fact, an elevation of plasma ACTH is the earliest and most sensitive indication of suboptimal adrenocortical reserve.
Symptoms and signs
- The main presenting symptoms of adrenocortical insufficiency are hyperpigmentation, weakness, fatigue (unusual tiredness), dizziness when standing up, loss of appetite, weight loss, anorexia, and gastrointestinal disturbances (nausea, diarrhea, stomach ache and vomiting). Many of these symptoms are aspecific and therefore the diagnosis is often delayed.
- Generalized hyperpigmentation of the skin and mucous membranes is one of the earliest signs of adrenocortical insufficiency. It is increased in sun-exposed areas and accentuated over pressure areas such as the knuckles, toes, elbows and knees. It is accompanied by increased numbers of black or dark-brown freckles. The classic hyperpigmentation of the buccal (cheek) mucosa and gums is preceded by generalized hyperpigmentation of the skin; adrenal insufficiency should also be suspected when there is increased pigmentation of palmar creases, nail beds, nipples, areolae, and perianal mucosa. Scars that have formed after the onset of ACTH excess become hyperpigmented, whereas older ones do not.
- An increase in gastrointestinal symptoms during an acute adrenal crisis may confuse the diagnosis by suggesting a primary intra-abdominal process.
- Hypotension (low blood pressure) is present in about 90% of patients and is accompanied by orthostatic (standing) symptoms and occasionally syncope (fainting).
- Salt craving occurs in 20% of patients.
- Severe hypoglycemia may occur in children. This finding is unusual in adults but may be provoked by fasting, fever, infection, or nausea and vomiting, especially in an acute adrenal crisis.
Treatment
The aim of the treatment of adrenocortical insufficiency is to produce levels of glucocorticoids and mineralocorticoids equivalent to those achieved in an age-related individual with normal hypothalamic-pituitary-adrenal function under similar circumstances. Patients require lifelong glucocorticoid and mineralocorticoid therapy.
Cortisol (hydrocortisone) is the glucocorticoid preparation of first choice. Cortisol in twice-daily doses gives satisfactory responses in most patients; however, some patients may require only a single morning dose, and others may require 3 doses to maintain wellbeing and normal energy levels. Insomnia is a side effect of glucocorticosteroid administration and can usually be prevented by administering the last dose at 4:00-5:00 pm.
Fludrocortisone us used for mineralocorticoid therapy. Because of the long half life of this agent, divided dose are not required. About 10% of patients with primary adrenocortical insufficiency can be managed with cortisol and adequate dietary sodium intake and do not require fludrocortisone.
Adrenal insufficiency inevitably leads to DHEA deficiency (the main precursor of sex steroid synthesis). Recent studies have pointed to the potential benefits of DHEA in doses of 25-50 mg/d orally in terms of improvement in well being.
