Urinary Free Aldosterone in Healthy People and in Patients with Primary Aldosteronism

Abstract

Urinary free aldosterone was measured with a sensitive radioimmunoassay. Mean values were .35 ± .15 μg/day in 11 normal persons and .95 ± .59, μg/day in 9 patients with primary aldosteronism, the difference being significant (p < 0.001). A good correlation was found between excretion of free aldosterone and excretion of 18-aldosterone glucuronide in the patient group (r = 0.81, p < 0.01). Urine specimens from such patients occasionally contain normal amounts of aldosterone. Thus, measurement of either urinary free aldosterone or of 18-aldosterone glucuronide gives values of equal significance. This is unlike the excretion pattern encountered when cortisol is secreted in excess, i.e., an increasing ratio of free-to-conjugated steroid with increasing secretion rates. The established difference in protein-binding between cortisol and aldosterone probably accounts for these different patterns of excretion.