Which laboratory finding is a hallmark of primary hyperaldosteronism?

In primary hyperaldosteronism, also known as Conn's syndrome, the adrenal glands produce excess aldosterone. This condition is characterized by specific laboratory findings, one of which is hypomagnesemia. Aldosterone promotes renal sodium reabsorption but also leads to increased excretion of potassium and magnesium through the urine. As a result, patients with primary hyperaldosteronism frequently exhibit low levels of magnesium in the serum.

High renin levels would typically be associated with secondary causes of hyperaldosteronism, wherein there is an underlying issue leading to increased renin secretion, like renal artery stenosis. Similarly, a hallmark feature of primary hyperaldosteronism is often low potassium levels due to the urinary excretion, making the presence of low potassium less specific for the disease compared to hypomagnesemia. Although serum sodium can be elevated due to the action of aldosterone in promoting sodium retention, it is not a specific finding for this condition since sodium levels can be influenced by various factors and do not directly indicate hyperaldosteronism.

Thus, hypomagnesemia stands out as a key laboratory finding that links directly to the excess production of aldosterone in primary hyperaldosteronism.