What is a hallmark finding in patients with metabolic acidosis due to CAH?

In metabolic acidosis due to congenital adrenal hyperplasia (CAH), particularly from the classic form caused by 21-hydroxylase deficiency, a hallmark finding is indeed low bicarbonate levels. In CAH, there is a deficiency in hormone production (cortisol and aldosterone), leading to an accumulation of steroid precursors, including 17-hydroxyprogesterone, which can cause increased production of androgens.

As the body is unable to produce sufficient cortisol, there is an impaired negative feedback mechanism on the hypothalamus and pituitary gland, resulting in elevated adrenocorticotropic hormone (ACTH) levels. This hormonal imbalance can lead to increased renal excretion of bicarbonate, contributing to a decreased serum bicarbonate concentration.

In addition to low bicarbonate levels, the condition often leads to compensatory respiratory mechanisms, such as increased ventilation to blow off carbon dioxide. Therefore, the low bicarbonate levels directly reflect the primary metabolic acidosis that occurs in patients with CAH. This finding is critical for diagnosis and management, highlighting the importance of monitoring serum bicarbonate in individuals suspected of having metabolic disorders such as CAH.