Antiadrenal Antibody

The reference range for this assay is < 1 U/mL.

Positive results from this assay (ie, ≥1 U/mL) indicate the presence of adrenal cortex autoantibodies in serum, a finding that is consistent with Addison disease. ^[1]

Specifics for the handling of specimens are as follows:

• Container - Plain red or SST

• Preparation - Transfer 1 mL serum to transport tube

• Temperature for storage and transport - Refrigerated

• Hemolyzed specimens are unacceptable

• Stability (from collection to initiation of testing [after separation from cells]) - Ambient temperature, unacceptable; refrigerated, 1 week; frozen, 6 months

Measurement of 21-hydroxylase-specific antibody

The 21-hydroxylase–specific antibodies are quantitated in the clinical laboratory by using radioimmunoassay (RIA). The calibrators, controls, and patient specimens are incubated overnight with iodine I¹²⁵ –labeled 21-hydroxylase. Specific antibodies, if present, bind to the 21-hydroxylase during this time. After overnight incubation, protein A is added to precipitate the antibodies present; the assay tubes are then centrifuged and the pellet is analyzed. The amount of radioactivity in the pellet is directly proportional to the amount of antibody contained in the specimen.

Description

Steroid synthesis is a very complex process that takes place in the adrenal glands and gonads. The pathway involves multiple enzymes and multiple enzymatic reactions. Steroid synthesis starts from cholesterol (low-density lipoprotein cholesterol [LDL-C]) and leads to production of three classes of specific adrenocortical steroid hormones: glucocorticoids, mineralocorticoids, and sex steroids. See the figure below.

Deletions or mutations of the genes encoding the enzymes involved in the synthesis or impaired function of the enzymes involved in the steroid hormones pathway (eg, autoantibodies) leads to deficiency of different hormones. ^[2]

In autoimmune adrenal disease, serum antibodies against several steroidogenic enzymes—including P450scc (CYP11A1, side-chain cleavage enzyme), P450c17 (CYP17, 17-alpha-hydroxylase), and P450c21 (CYP21A2, 21-hydroxylase)—are present. These enzymes play a role in side-chain cleavage and hydroxylation of steroids. Of these, most commonly, patients with autoimmune adrenal disease have autoantibodies directed against the microsomal 21-hydroxylase. Therefore, laboratory assessment of these types of antibodies is clinically very useful for the diagnosis of autoimmune adrenal disease (eg, Addison disease). ^{[3, 4]} The presence of antiadrenal autoantibodies in serum is characteristic of autoimmune Addison disease (ie, chronic primary adrenal insufficiency, resulting from autoimmune destruction of the adrenal cortex). ^{[1, 5]}

A study by Wolff et al indicated that individuals with autoimmune Addison disease have long-term positivity for 21-hydroxylase–specific antibodies, with the investigators finding that more than 90% of the study’s patients were still positive for the antibodies 30 years after their disease diagnosis. According to the report, indices for the antibodies are impacted by disease duration, sex, whether the Addison disease is isolated or part of APS type I or II, and human leukocyte antigen–risk genotype. ^[6]

Indications/Applications

Antibodies to 21-hydroxylase are markers of autoimmune Addison disease, whether this condition is occurring alone or in conjunction with other autoimmune endocrine diseases as part of type I or type II autoimmune polyglandular syndrome (APS). These antibodies may be present even before any reduction in endocrine function is apparent. Patients with autoimmune adrenal insufficiency commonly have antibodies against other endocrine glands, whereas healthy patients rarely do. More than 50% of these patients have high serum antithyroid peroxidase antibody levels, and nearly 50% have overt hypothyroidism. Many more of these patients exhibit subclinical hypothyroidism (evidenced by elevated thyroid-stimulating hormone [TSH] and normal thyroxine concentrations) and are at risk for subsequent development of overt hypothyroidism (Schmidt syndrome). ^{[7, 8]}

Autoimmune adrenal disease can be conveniently divided into phases. Initially, the adrenals may be enlarged, and extensive lymphocytic infiltration may be noted. In long-standing disease, however, the glands are small and may be hard to find. A thickened, fibrotic capsule is observed, with complete destruction of the cortex (although a few small clusters of adrenocortical cells surrounded by lymphocytes may remain) and relative sparing of the medulla. Only after at least 90% of the cortex has been destroyed does adrenal insufficiency clinically manifest.

Considerations

In patients with normal adrenal function who have other autoimmune endocrine diseases, the prevalence of antiadrenal autoantibodies in serum is low (2%), the exception being those with hypoparathyroidism, in whom the figure is substantially higher (16%). ^{[9, 10]}

Autoimmune adrenal insufficiency may be either familial or nonfamilial (acquired); when occurring alone, rather than as part of APS type I or II, it is somewhat less likely to be familial. About half the patients who have adrenal insufficiency as part of APS type I or II have affected family members, compared with only about one third of patients who have autoimmune adrenal insufficiency alone.

As the enzyme 21-hydroxylase is involved in the metabolism of both mineralocorticoids (aldosterone) and glucocorticoids (cortisol) (see figure above), impaired 21-hydroxylase function due to the binding antibodies can lead to deficiency in both of these types of hormones. On the other hand, patients have an elevated serum concentration of 17-α hydroxyprogesterone, progesterone, 17-α hydroxypregnenolone, and pregnenolone. Adrenal androgens, dehydroepiandrosterone sulfate (DHEA-S), DHEA, and 3-α androstanediol (a metabolite of androgens) are also often elevated in these patients.

Women might present with hirsutism (mild), polycystic ovarian syndrome (PCOS) and primary and secondary amenorrhea. The adrenals might become hyperplasic and the patient might have low serum sodium and chloride levels and hyperkalemia (salt-wasting forms). ^[2]