Primary Polydipsia

Today, we reviewed an excellent case of a middle aged man with a history of polysubstance abuse and schizophrenia who was found to have hyponatremia, ultimately diagnosed with primary polydipsia.

Learning Points:

  1. Hyponatremia can be classified according to volume status into three categories including: hypovolemic, euvolemic, hypervolemic hyponatremia. (There are other ways to classify too, including by serum ADH!)
  2. Each category can be subdivided based on either urine Na for hypo/ervolemic hyponatremia or urine osms for euvolemic hyponatremia.
  3. Urine osm < 100 is consistent with primary polydipsia and urine osm >100 is consistent with SIADH, hypothyroidism or glucocorticoid deficiency. Our patient's urine Osm was 84, consistent with primary polydipsia.

Dr. Larson had several pertinent take-home points including:

  1. Primary polydipsia is common in psychiatric illness (schizophrenia being the highest) and 10-20% episodes of acute psychosis manifest some degree of polydipsia.
  2. There is an interplay between antipsychotic use, primary polydipsia and hyponatremia such that pharmacologic therapy can cause an SIADH type picture, while psychosis/stress may cause primary polydipsia.
  3. Urine studies are helpful in differentiating which is the primary mechanism.