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.
Excessive corticosteroid medications or too little sodium and potassium in the blood (hyponatremia, hypokalemia) as occasionally occurs
on high doses of diuretics can both lower your pet's blood
osmolality.