A patient with Addison's disease is expected to have which electrolyte abnormality?

Addison's disease, or primary adrenal insufficiency, occurs when the adrenal glands do not produce enough cortisol and often also aldosterone. Aldosterone is crucial for regulating sodium and potassium levels in the body. In Addison's disease, due to low levels of aldosterone, there is decreased reabsorption of sodium in the kidneys. Consequently, sodium levels in the blood drop, leading to hyponatremia (low sodium).

At the same time, the lack of aldosterone results in poor excretion of potassium, causing the potassium levels in the blood to rise, leading to hyperkalemia (high potassium). Thus, the typical electrolyte abnormality expected in a patient with Addison's disease is low sodium and high potassium. This imbalance has critical implications for maintaining fluid balance and normal heart function, and it often presents with characteristic symptoms such as weakness, fatigue, and, in severe cases, cardiac issues.

In summary, the presence of low sodium and high potassium accurately reflects the underlying pathophysiology of Addison's disease due to a deficiency in aldosterone secretion from the adrenal glands.