From "Diagnosis of Cushing's Syndrome in the Modern Era" by L.K. Nieman, 2018:
A urine free cortisol (UFC) result may incorrectly exclude hypercortisolism in patients with a glomerular filtration rate less than 30 mL/min but might falsely diagnose hypercortisolism in individuals drinking more than 5 L of fluid daily. In the former situation, UFC should not be chosen as a screening test." In the latter group of individuals, urinary cortisone and cortisol both increase, so that the effect may be more accentuated in immunoassays in which cortisone cross-reacts with the antibody. Thus, 2 ways of addressing this are to use tandem mass spectroscopy assays (or immunoassays with minimal cross-reactivity) and to ask patients to restrict fluids to 2 L/d.
Why is that so? Is it because the body would naturally need to produce more cortisol in order to expel all this water?