| Figure 1. Schema of adrenal steroidogenesis. (84) |
Cortisol is synthesized under the trophic control of adrenocorticotropic hormone (ACTH), forming a negative feedback loop in which high serum cortisol centrally inhibits and low serum cortisol stimulates release of ACTH, which defines the hypothalamic-pituitary-adrenal axis. Net ACTH release has basal, diurnal, and stress-induced components. The pulsatile release of ACTH is in turn modulated by corticotropin releasing hormone (CRH), while the central nervous system determines the hypothalamic set point for the expected plasma cortisol level. A deficiency of 21-hydroxylase impairs production of cortisol resulting in increased pituitary secretion of ACTH. Chronic elevations of ACTH levels stimulate the accumulation of precursor steroids in the impeded pathways and cause excessive steroid synthesis in other adrenal biosynthetic pathways unaffected by the enzyme deficiency.
Steroids in 21-hydroxylase deficiency
Deficiency of 21-hydroxylase occurs in three forms: 1) simple virilizing, 2) salt-wasting, and 3) nonclassical. The simple virilizing and salt-wasting forms of 21-hydroxylase deficiency are characterized by excess adrenal androgen biosynthesis in utero, which causes prenatal virilization of the genetic female and postnatal virilization of both boys and girls. Biochemically, the conversion of 17a-hydroxyprogesterone (17-OHP), the main substrate of the 21-hydroxylase enzyme, to 11-deoxycortisol in the pathway of cortisol synthesis is impaired (Figure 1). Baseline and ACTH stimulated serum levels of cortisol precursors may be extremely elevated in untreated patients, principally, 17-OHP. The 21-hydroxylase enzyme defect can also impair the conversion of progesterone to 11-deoxycorticosterone (DOC) in the pathway of aldosterone synthesis, as occurs in the salt-wasting form. Aldosterone production is not sufficient for sodium reabsorption by the distal renal tubules causing low serum sodium, high serum potassium, in addition to increased levels of adrenal androgen and cortisol precursors seen in simple virilizers. In salt-wasting CAH, both newborn boys and girls are subject to early, life-threatening, salt-wasting crises within the first few weeks of life.
Hormonal diagnosis of 21-hydroxylase deficiency is established by comparison of baseline and ACTH stimulated serum levels of 17-OHP. 1 A nomogram provides hormonal standards for assignment of the 21-hydroxylase phenotype (Figure 2). Because of the diurnal variation in l7-OHP, an early morning serum concentration of l7-OHP may be useful as a screening test for genotyping 21-OHD. ACTH stimulation, however, remains the most definitive hormonal diagnostic test. An ACTH stimulation test should not be performed during the initial 24 hours of life as samples from this period are typically elevated in all infants and may yield false-positive results.
| Figure 2. Nomogram for hormonal diagnosis of 21-hydroxylase deficiency relating baseline to ACTH-stimulated serum concentrations of 17-OHP. Scales are logarithmic. A regression line for all data points is shown. |
Nonclassical CAH, sometimes called late-onset, occurs when there is only a mild deficiency of enzyme 21-hydroxylase. These patients do not waste salt, and females are not virilized at birth. Nonclassical 21-hydroxylase deficiency may present at any age postnatally, with patients exhibiting moderately high androgens and stimulated levels of 17-OHP. 2
The various clinical and biochemical features associated with the different forms of 21-hydroxylase deficiency are presented in Table 1.
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