Blood urea nitrogen (BUN) is an indication of renal (kidney) health. The normal range is 1.8-7.1 mmol/L or 6–20 mg/dL.
The main causes of an increase in BUN are: high protein diet, decrease in glomerular filtration rate (GFR) (suggestive of renal failure) and in blood volume (hypovolemia), congestive heart failure, gastrointestinal hemorrhage, fever and increased catabolism. Hypothyroidism can cause both decreased GFR and hypovolemia, however BUN-to-creatinine ratio has been found to be lowered in hypothyroidism and raised in hyperthyroidism.
The main causes of a decrease in BUN are severe liver disease, anabolic state, and syndrome of inappropriate antidiuretic hormone.