One measured fluid, electrolyte, and renal indices of hydration over eleven days of caffeine consumption in human subjects, finding that doses of up to 6 mg caffeine per kilogram of body weight had no effect on body mass, urine osmolality (urine concentration), urine specific gravity (concentration of excreted materials in urine), urine color, urine volume, sodium excretion, potassium secretion,
creatinine content, blood urea nitrogen (forms when protein breaks down), and serum levels of sodium and potassium.
However, the only large study comparing it with monohydrate showed no difference in performance or muscle creatine
content, and it found pretty similar increases in
creatinine between the two.