I have hemoconcentration with low normal sodium levels, and mildly elevated creatinine and bun. my urine sodium is 80 mmol/l and urine osmality is 900 mosmol/kg. i get bad muscle cramps. i always drink plenty of water. what could be causing this?

The term hemoconcentration is not appropriately used with the provided info. Urine sodium of 80 mmol/L rules out pre-renal conditions which may cause low serum sodium. If your serum sodium is low but urine osmolality is high then you have problem of diluting the urine. You should see a nephrologist who should be able to address your questions in details. Take care.

You don't say what your hematocrit & hemoglobin are, but I'm not convinced there's true hemoconcentration here. Are you exercising often& vigorously enough to develop muscle cramping? Could magnesium deficiency account for your muscle cramping? Is it from low serum sodium from excess water intake? Are you being seen by a nephrologist and, if so, what is the cause of your elevated BUN& creatinine?