Disorders of the Adrenal Gland

Embryology

Normal adrenal function is critically important for maintenance of intrauterine homeostasis, promotion of organ maturation, and adaptation to extrauterine life. The dual embryologic origin of the human adrenal gland results in an outer adrenal cortex and an inner adrenal medulla; each part secretes different vital hormones critical to fetal development. Embryologically, the adrenal cortex develops from the coelomic mesoderm of the urogenital ridge, whereas the medulla arises from neural crest tissue in the adjacent sympathetic ganglion at celiac plexus level. During the fifth week of fetal development, mesothelial cells from the posterior abdominal wall between the root of the bowel mesentery and developing mesonephros proliferate and form the primitive adrenal cortex. In the sixth week, a second wave of mesothelial cells surrounds the primitive cortex and later forms the adult or definitive cortex. By 8 weeks of gestation, the cortical mass separates from the rest of mesothelial tissue and becomes surrounded by connective tissue. This separation divides adrenocortical and gonadal primordium. Chromaffin cells, which originate from neural crest, migrate toward the adrenal cortex around this time and gradually invade the medial aspect of the cortical tissue along its central vein to gain central position, forming the adrenal medulla. Fig. 84.1 summarizes the embryologic origin of the adrenal gland. By 8 to 9 weeks of gestation, the adrenal gland is encapsulated and contains an outer “definitive” zone where glucocorticoids and mineralocorticoids are synthesized and a larger inner “fetal” zone where dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) are produced and then subsequently converted to estriol by the placenta.

Postnatally, the fetal or primitive zone of the adrenal gland rapidly involutes to disappear by approximately 6 months of age. Zonation of the cortex, zona glomerulosa, and fasciculata is present at birth, but full differentiation into three separate zones occurs much later at approximately 3 years of age when zona reticularis development takes place. Fig. 84.2 depicts these changes in the adrenal gland from 9 weeks of gestation through childhood.

Morphology

The adrenal glands are bilateral structures located above the kidneys in the retroperitoneum area. At birth, the adrenals are approximately one-third the size of neonatal kidneys, weighing 8 to 9 g, and are 10- to 20-fold larger than the adult glands relative to body weight (0.4% vs. 0.01%). In the third trimester and the first 3 months after birth, the glands predominantly consist of cortex, where active production of glucocorticoids, steroid precursors, estrogens, and progesterone takes place. Ultrasonographically, the neonatal adrenal gland characteristically has a thin reflective core surrounded by a thick transonic zone. The gland subsequently decreases in size as the active fetal cortex regresses to reach approximately 8% of the kidney size in adulthood.

Histologically, the fetal adrenal cortex consists of a small outer definitive zone, which appears to produce few adrenal steroid hormones until late gestation, and a larger inner fetal zone that produces adrenal steroid hormones throughout gestation. In addition, there is a transitional zone where cortisol production takes place toward the end of fetal development. At birth, the large fetal zone of the fetal adrenal involutes and disappears by 6 months of age. Concurrently, the definitive zone together with the transitional zone develops into the fully differentiated zona glomerulosa and fasciculata by the age of 3 years. The zona reticularis begins to develop only after 4 years of age and may not be fully differentiated before the age of 15 years. In an adult adrenal gland, these three distinctive zones lie adjacent to one another. The zona glomerulosa is located immediately below the capsule, the zona fasciculata is in the middle, and the zona reticularis is the innermost zone next to the medulla.

Adrenal Functions

A cascade of adrenal steroidogenesis in the adult is shown in Fig. 84.3 . Three major pathways of mineralocorticoid, glucocorticoid, and androgen synthesis take place mainly in the glomerulosa, fasciculata, and reticularis zones of the cortex, respectively. Aldosterone is the main mineralocorticoid regulating sodium and fluid volume homeostasis. Aldosterone is under the control of the rennin–angiotensin system and blood potassium concentrations. The principal glucocorticoid in humans is cortisol and has a wide range of roles in regulating body functions, from carbohydrate metabolism, immune system, and acute and chronic stress response to musculoskeletal metabolism. Cortisol production is regulated through a negative feedback loop involving hypothalamic corticotropin-releasing hormone (CRH) and pituitary adrenocorticotropic hormone (ACTH). Adrenal androgens have an age-specific secretion profile with an increase at adrenarche, which occurs 2 years before the time of puberty, and then a gradual decrease with aging until andropause. The regulatory mechanism behind normal adrenal androgen production is largely unknown but involves ACTH to some extent.

In the fetal adrenal gland, steroidogenic enzymes are found as early as 7 weeks' gestation. At 8 weeks' gestation, the fetal adrenal gland produces cortisol under ACTH control. A transient expression of 3β-hydroxysteroid dehydrogenase type 2 (3β-HSD2) during this critical time from 7 to 12 weeks' gestation allows the fetal adrenal gland to produce cortisol. Activation of 3-HSD2 serves principally to prevent virilization of the female genital anlage that would otherwise result from overwhelming amounts of DHEAS and its downstream androgen metabolites. It is believed that a transient peak of cortisol during this time suppresses the fetal hypothalamic–pituitary–adrenal (HPA) axis, keeping DHEAS production at a low level. By the end of the first trimester, cortisol secretion from the fetal adrenal gland begins to wane as a result of a decrease in 3β-HSD2 expression, thus decreasing HPA axis suppression with resultant increased DHEAS secretion. During the second and third trimesters, the fetal adrenal gland secretes abundant amounts of DHEA and its sulfated derivative DHEAS, earning the term androgen factory . The rate of steroid secretion by the fetal adrenal glands may be fivefold that of the adult adrenal glands at rest. The placenta can also convert DHEAS back to DHEA by a sulfatase enzyme (i.e., arylsulfatase). These adrenal steroids serve as precursors for 17,20 lyase activity of the P450C17 enzyme for androgen production and subsequent estrogen production ( Fig. 84.4 ). In addition, DHEAS is oxidized in the fetal liver to a 16α-hydroxylated derivative, which is converted by the placenta to estriol by the same set of enzymes as in estradiol synthesis.

The physiology of human pregnancy involves a continuous supply of relatively increased amount of estrogens. In near-term human pregnancy, the rate of estrogen production increases strikingly, reaching concentrations 1000-fold greater than that of nonpregnant women. During early gestation, the estradiol required to maintain pregnancy is provided by the corpus luteum of the maternal ovary. But after 8 weeks' gestation, the fetoplacental unit synthesizes most of the estradiol required to maintain pregnancy.

DHEAS is the main steroid secreted by the fetal adrenal cortex from mid-gestation onward. The activity of 3β-HSD2 controls fetal cortisol synthesis. By the end of pregnancy, fetal cortisol is required in preparation for parturition (i.e., lung maturation or surfactant production) and could have a role in triggering parturition, as shown in other species. Maternal cortisol cannot normally reach the fetus because it is oxidized to cortisone, an inactive steroid, by placental 11β-hydroxysteroid dehydrogenase type 2. When pregnancy approaches term, 3β-HSD2 expression increases again and remains high, allowing increased cortisol secretion. In a child, 3β-HSD2 secretion is high in the adrenal gland until adrenarche, when 3β-HSD2 activity decreases again to allow an increase in DHEA and its downstream androgen metabolite secretion, which gives rise to the development of pubic and axillary hair.

Control of Glucocorticoid and Mineralocorticoid Production

Two distinct regulatory circuits control adrenal glucocorticoid and mineralocorticoid secretion. The HPA axis determines the set point for circulating glucocorticoid (cortisol) concentration. The neuropeptide CRH (or factor) and arginine vasopressin (AVP) are synthesized in the hypothalamic paraventricular nucleus and released into the hypophysial portal circulation at the median eminence in response to stress and, beginning at approximately 6 months of age, to circadian cues. These neuropeptides stimulate the release of ACTH from the anterior pituitary corticotrophs. CRH is the primary stimulator of ACTH, while AVP amplifies the effect of CRH. ACTH released into the systemic circulation augments adrenocortical secretion of cortisol and DHEA by acting on the ACTH receptor, a member of the melanocortin receptor family. The ACTH receptor is present on steroidogenic cells of the fetal zone and transitional zone of the fetal adrenal as well as the adult adrenal cortex. The resulting increase in plasma cortisol concentration limits further the release of hypothalamic neuropeptides and ACTH by negative feedback through glucocorticoid receptors at the central nervous system and pituitary sites. As a corollary, if glucocorticoid production is impaired by intrinsic adrenal dysfunction, then neuropeptide CRH and ACTH release are augmented.

The components of the HPA axis are present early in human development. As detailed earlier, the fetal adrenal gland begins to develop at 4 weeks’ gestation, when initial evagination of the pituitary primordium occurs. ACTH-producing pituitary cells can be detected at 7 weeks’ gestation, and an intact hypophysial portal vascular system is present by 12 weeks’ gestation. Nerve terminals containing CRH can be detected in the hypothalamus by approximately 16 weeks’ gestation. Virilization caused by increased production of adrenal androgens in females with congenital adrenal hyperplasia (CAH) occurs before 12 weeks’ gestation; therefore, ACTH-producing corticotrophs must undergo cortisol-mediated feedback modulation at the initial stages of hypothalamic–pituitary development.

Mineralocorticoid (aldosterone) release by the zona glomerulosa of the adrenal cortex is determined by the renin-angiotensin system with acute modulation to a lower extent by ACTH as well. Decreases in vascular volume result in increased secretion of renin by the renal juxtaglomerular apparatus. Renin, a proteolytic enzyme, cleaves angiotensinogen to angiotensin I, a biologically inactive decapeptide. Angiotensin I is then cleaved and activated by angiotensin-converting enzyme in the lung and other peripheral sites to angiotensin II. Angiotensin II and its metabolite angiotensin III possess vasopressor and potent aldosterone secretory activity. Although angiotensin II receptors are present on cells of the definitive zone at 16 weeks' gestation, significant aldosterone production by the fetal adrenal gland does not occur until the third trimester of pregnancy.

Molecular Basis of Adrenal Development

Several transcription factors are critically important for normal adrenal development. Two related transcription factors have emerged as key regulators of adrenal development: the nuclear receptor DAX-1 (dosage-sensitive sex reversal-adrenal hypoplasia congenita [AHC] critical region on the X chromosome gene-1, encoded by NROB1 / AHC ) and steroidogenic factor-1 (SF-1, encoded by NR5A1 , also known as AD4BP). Sf-1 ( Nr5a1 ) knockout mice lack adrenal glands and gonads, and subsequent identification of NR5A1 mutations in humans with adrenal insufficiency confirms the essential role of SF-1 in development. Nr5a 1 expression is found in the early urogenital ridge of the mouse in cells that give rise to both the bipotential gonad and adrenal cortex. Expression of Nr5a1 remains high throughout embryogenesis, the postnatal period, and adult life.

DAX-1 is an orphan nuclear receptor that colocalizes with SF-1 in the cells of adrenal glands, gonads, gonadotropes, and ventral–medial lateral nucleus of the hypothalamus. Deletion of NROB1 results in AHC. Although the exact role of DAX-1 in adrenal development is not known, it has been shown to interact with SF-1. Normally, DAX-1 recruits the nuclear corepressor N-CoR to SF-1 and represses SF-1. Similarly, the Wilms tumor suppressor gene ( WT1 ) protein has been shown to interact with SF-1. WT1 encodes 24 different protein isoforms that act as transcription factors. Wt1 is detected in the urogenital ridge of the mouse embryo but is not detected in adult or fetal adrenals. Mutations in Wt1 have resulted in abnormal development of the adrenal in the mouse but have not been clearly correlated with abnormal human adrenal development. WT1 has been found though to be expressed in the fetal adrenal gland.

Assessing Adrenal Function in the Newborn

The adrenal cortex plays a major role in the newborn postnatal adaptation, and significant evolution of adrenal steroid production occurs over the first days and months of life. During interpretation of the newborn adrenal steroidogenic function, special attention must be paid to age-related changes in adrenal steroid intermediates, circulating cortisol, and aldosterone concentrations that reflect ongoing adrenal maturation. For example, until at least 1 month postnatally, a large proportion of cortisol and its metabolites is excreted as sulfate esters. This sulfation may serve to inactivate a number of circulating cortisol metabolites during fetal and neonatal life.

Immediately after birth the third trimester fetal zone that once was the predominant component of the adrenal cortex in the fetus and preferentially produced DHEA and DHEAS starts to reduce in size. This rapid loss of the fetal zone during this period results in a dramatic fall of the circulating DHEA concentration over the first week to 1 month postnatally. The variable pattern of decline in the ensuing weeks probably reflects variation in remodeling of the fetal zone and emergence of the zona fasciculata of the definitive zone, the latter being a feature of an adult cortex. In addition to diminished 3β-HSD2 activity, preterm infants have sustained elevations in 17-hydroxyprogesterone and the 17-hydroxyprogesterone-to-cortisol ratio, suggesting a reduction in 21-hydroxylase activity. Because blood-spot 17-hydroxyprogesterone concentration is used for newborn screening of CAH in many states, many preterm infants initially have an abnormal test result. Subsequent follow-up testing is then required to determine whether CAH is present. Plasma aldosterone concentrations tend to be higher in preterm infants than in term infants, both of which in turn are higher than in older children and adults.

Cortisol has a critical role in maintaining homeostasis in response to stress. Relative adrenal insufficiency occurs when the HPA axis produces less than adequate cortisol for the degree of illness or stress. Immaturity of the adrenal gland and the HPA axis of the premature newborn infant suggest a rationale for why preterm infants are at increased risk of cortisol insufficiency. Clinicians are commonly faced with critically ill infants who have cardiovascular insufficiency with hypotension, a condition that has been associated with adverse consequences. The question often arises as to whether these manifestations reflect underlying glucocorticoid insufficiency. There is increasing evidence that relative adrenal insufficiency may be a cause of hemodynamic instability and hypotension in the critically ill newborn, but there is definitely a paucity of data in this population.

Random plasma cortisol measurement is often inadequate to answer this question because the majority of critically ill newborns have low cortisol and ACTH values without the expected increase in response to critical illness. Infants also do not exhibit diurnal variation of ACTH and cortisol until 6 months of age. Thus, obtaining two or more samples of cortisol may be informative. Response to exogenous ACTH (cosyntropin) is usually normal, suggesting that the inadequate response to critical illness in these newborns does not result from adrenal dysfunction but arises from some other components of the HPA axis. Interestingly, in extremely low birthweight infants (500 to 999 g) low cortisol concentrations were not predictive of adverse short-term mortality and morbidity. In contrast, high basal cortisol levels were associated with severe intraventricular hemorrhage, and extremely elevated values were associated with morbidity and death.

Data associating treatment of adrenal insufficiency with outcomes in the term newborn are limited, and there have been no studies on outcomes beyond the immediate neonatal period. Nonetheless, no adverse events have been attributed to glucocorticoid treatment based on a relatively small number of study subjects. Currently there is insufficient evidence to support the routine use of glucocorticoids in critically ill newborns. On encountering an infant with vasopressor-resistant hypotension accompanied by signs of cardiac hypofunction, the clinician must consider the risk-to-benefit ratio before arriving at the appropriate management. Therapeutic trials with hydrocortisone at the dose of 1 mg/kg of body weight have been suggested and can be discontinued if there is no clinical improvement or if the pretreatment cortisol level is later observed to be greater than 15 µg/dL. A meta-analysis of the use of hydrocortisone for hypotension and vasopressor dependence in preterm infants showed improvement in blood pressure and less need for vasopressor, but the clinical benefit is unknown, and long-term effects of hydrocortisone use are not known. Special attention should be paid to the premature newborn concurrently receiving indomethacin because the combination of hydrocortisone and indomethacin is associated with spontaneous gastrointestinal perforation.

Steroidogenic Defects Caused by Adrenal Enzyme Deficiency

CAH refers to a family of inherited adrenal gland disorders in which defects occur in one of the enzymatic steps required to synthesize cortisol from cholesterol; therefore, impaired cortisol synthesis is the cornerstone shared by all forms of CAH. The pathway of steroidogenesis in the adrenal cortex is illustrated in Fig. 84.3 . Five forms of CAH with autosomal recessive mode of inheritance are summarized in this section.

21-Hydroxylase Deficiency

Management

Hormone replacement therapy with corticosteroids is used to correct the deficiency in cortisol secretion, which will in turn suppress ACTH overproduction and subsequent stimulation of the androgen pathway. Hormone replacement prevents further virilization, allowing normal growth and onset of puberty. The initial dose of hydrocortisone required is usually 15 to 20 mg/m ² per day divided into three doses per day but may be even higher in the newborn. The dose can then be decreased to 10 to 15 mg/m ² per day after there has been initial suppression of the ACTH–adrenal axis. It is important that the appropriate balance be maintained to avoid hypercortisolism, which can result in Cushing syndrome and suppression of linear growth. Attempts to suppress 17-OHP levels to normal will inevitably result in iatrogenic Cushing syndrome. Hormonal control can be difficult to achieve in some cases, and adrenalectomy in the past had been offered as an extreme alternative therapeutic option in select patients. Patients with salt-losing CAH have elevated plasma renin activity and require mineralocorticoid replacement with 0.05 to 0.4 mg/day of 9α-fludrocortisone. Neonates have mineralocorticoid resistance as evidenced by higher levels of aldosterone present in normal infants and thus the required dose of 9α-fludrocortisone is higher in infants than in adults. In infancy, most patients also require an oral salt supplement as in other forms of primary adrenal insufficiency. There is no parenteral formulation for mineralocorticoid. Therefore, if 9α-fludrocortisone cannot be given orally, hydrocortisone at higher doses can be given IV along with sodium chloride. A dose of 20 mg of hydrocortisone will provide the equivalent of 0.1 mg 9α-fludrocortisone.

With the advent of newborn screening, infants are diagnosed with CAH prior to a salt-wasting crisis, and thus it is not possible to distinguish the infant with the 75% chance of having salt-wasting CAH without genetic testing. Because females with classic 21-OHD CAH are born with ambiguous genitalia caused by the production of excess androgens in utero, corrective surgery is contemplated. However, before surgical restoration is considered as a form of treatment in patients with ambiguous genitalia, consultation with the patient's parents, psychologist, pediatric endocrinologist, and pediatric urologist is essential.

Prevention of prenatal virilization in affected females is possible with a prenatal diagnosis and treatment program. The disease can be diagnosed prenatally through molecular genetic analysis of fetal DNA. Prenatal treatment by dexamethasone administration to the pregnant mother carrying an at-risk fetus before 6 weeks post conception is effective in reducing virilization in the genetic female, making postnatal female restorative surgery less likely to be necessary and thereby avoiding potential impairment of sexual function ( Fig. 84.5 ). At 7 to 10 weeks, cell-free DNA testing may allow for discontinuation of the dexamethasone if an XY fetus is identified or, if using massive parallel sequencing (MPS) of cell-free fetal DNA, an XX fetus is found to not have CAH. However, the technique of MPS for CAH is not readily available as routine care. Dexamethasone may also be discontinued after diagnosis is made by fetal DNA analysis obtained from chorionic villus sampling at 10 to 12 weeks' gestation in unaffected female or male fetuses. Some believe there are accurate, compelling data from the largest human studies indicating the benefit of prenatal treatment and that it is safe in the short term for both the fetus and the mother. Some preliminary data from long-term studies also support these results, although other long-term follow-up studies are still under way. In contrast, studies from Sweden found that children without CAH treated prenatally with dexamethasone had decreased verbal working memory and decreased self-perception of scholastic ability. Those with CAH had decreased verbal processing speed that normalized though when adjusted for intelligence quotient. A more recent review found that there is a negative effect on executive function and on social behavior. Long-term cardiovascular, renal, and metabolic risk have yet to be determined. Thus, the ethics of prenatal dexamethasone therapy remains controversial because of unknown long-term outcomes and because seven fetuses were treated unnecessarily to prevent virilization in one fetus. Dexamethasone therapy also incurs maternal risk. Consequently, all endocrine societies consider prenatal dexamethasone therapy for 21-OHD to be experimental and that it should only be performed in a research environment with long-term outcome studies, as the risk for long-term effect is not known.

11β-Hydroxylase Deficiency