Electrolytes and Endocrine Practice Test 2026 by Mark Klimek – Your Comprehensive All-In-One Study Guide for Exam Success!

Measuring how the kidney handles water in the setting of low serum osmolality is what separates SIADH from primary polydipsia. In SIADH, antidiuretic hormone is secreted inappropriately, so the urine remains concentrated even though the blood is dilute. This shows up as a high urine osmolality and a high urine sodium because the kidneys continue to excrete sodium while retaining water, keeping the person euvolemic. In primary polydipsia, there’s excessive water intake, so the kidneys dump free water, producing very dilute urine with low urine osmolality and variable urine sodium depending on intake and volume status.

So testing urine osmolality and urine sodium, alongside confirming hypotonic hyponatremia with serum osmolality, gives the clearest distinction between these two scenarios. Serum sodium alone can be low in both conditions, and plasma ketone or BUN levels don’t reliably separate them.