Urea is produced in the liver as a result of endogenous and dietary protein metabolism. The BUN increases with renal disease, but is also influenced by diet or state of hydration . This is not a good indicator of renal function in the cow, as urea is utilized in the rumen for protein synthesis. Creatinine is produced in muscle when creatine is converted to creatinine. Creatinine is freely filtered through the glomerulus but is not reabsorbed by the tubules.
Dogs have some tubular secretion of creatinine.
Creatinine increases as the GFR decreases. ADH increases the permeability of the renal tubular epithelium to water, causing water resorption by the tubules and increasing the concentration of urine. In cases of medullary washout, gradual restriction of water can be tried, in order to allow reestablishment of a hypertonic renal medulla. If the concentration of urine is not seen with the water deprivation test, exogenous ADH is given for an ADH response test. If the animal can't concentrate urine after the deprivation test, but can concentrate with exogenous ADH, the diagnosis is pituitary diabetes insipidus.