Today we discussed an interesting case of a patient with hypercalcemia, presenting with polyuria. Remember, polyuria has a specific definition - technically >3L/24h. However, we must remember that a volume less than this may be inappropriately high (ie the patient is still "polyuric") if the patient is volume deplete. There is relatively limited differential diagnosis: the problem is either too much solute or too much volume. See here for a previous post on polyuria.
Diabetes insipidus can be central or nephrogenic, depending on whether this is insufficent production/release of ADH, or inadequate response to ADH at the level of the kidney. Central diabetes is related to pituitary dysfunction, such as due to tumour, infiltration, inflammation, or trauma. Nephrogenic DI is often related to lithium use, congenital, or (as in our case) due to hypercalcemia.
Testing requires: urinalysis, 24hour urine for volume, urine electrolytes, and osmolarity, serum lytes and serum osmolarity. DI is suspected with a high serum osmolarity but low urine osmolarity (ie an inappropriately dilute urine). The response to DDAVP, often done with or without a water deprivation test (arguably not needed if they are already hypernatremic/high serum Osm), indicates the difference between central and nephrogenic DI. In central DI, the kidney should respond by concentrating urine (ie rising urine osmolarity), while in nephrogenic DI there will be no change with exogenous DDAVP.
See here for a previous post on hypercalcemia.
