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The measurement of antibodies against 21-hydroxylase is used in the investigation of patients with Addison's disease to control the autoimmune origin of the disease.

21-hydroxylase is a microsomal enzyme in the cytochrome P-450 complex that plays a key role in steroid biogenesis in the adrenal cortex. 21-Hydroxylase is responsible for the conversion of progesterone and 17-hydroxyprogesterone into deoxycorticosterone and 11-deoxycortisol. In patients suffering from Addison's disease who are positive for anti-adrenal antibodies against immunofluorescence, these antibodies were found to be directed against 21-hydroxylase (21-OH). Serum antibodies in patients with Addison's disease inhibit the activity of the enzyme in vitro, so that progesterone is no longer converted to deoxycorticosterone.

These antibodies are capable of causing adrenal insufficiency due to the inhibition of 21-hydroxylase.

Antibodies against 21-hydroxylase are not present in patients with secondary Addison's disease after tuberculosis or leukodystrophy. These antibodies are found in most patients with anti-adrenal antibodies. There is a correlation between the presence of anti-adrenal antibodies and anti-21-hydroxylase antibodies, suggesting that 21-hydroxylase is the major autoantigen for these antibodies.

In Addison's disease, antibodies against 21-hydroxylase tend to decrease over time and are likely to disappear completely in some patients after complete destruction of the adrenal cortex.

Antibodies against 21-hydroxylase are found in most patients with type I and type II autoimmune syndrome.

Antibodies against 21-hydroxylase have been found in a small proportion (1-4%) of patients with:

  • Chronic idiopathic hypothyroidism
  • Insulin-dependent diabetes
  • Hashimoto's thyroiditis
  • Graves disease

In these patients, adrenal insufficiency (Addison's disease) occurs after a latency period that is shorter in children than in adults. Therefore, these antibodies could be considered as an early marker of adrenal involvement, a condition that may become apparent in the coming months or years.

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