Why has the utility of TTKG as an assessment of aldosterone secretion and action been questioned?
TTKG was thought to represent the activity of aldosterone on the basis of an assumption that there is no appreciable reabsorption of osmoles downstream from the cortical collecting duct. This assumption has been found to be invalid after the discovery of the fact that urea is actively absorbed in the medullary collecting duct, a process that is regulated by vasopressin and aids in the secretion of potassium. Thus, TTKG represents potassium excretion due to both aldosterone and vasopressin action, and its utility in the diagnosis of hyperkalemia has been questioned.
