Take-home pearl: if thinking about free water restriction for hyponatremia, it will only correct if the patients free water intake < EFWC (estimated free water clearance). As a rule of thumb, if urinary [ Na + K] is less than plasma [Na], then free water restriction will be effective in correcting the hyponatremia.
Other teaching points from the case:
- SIADH: Remember that diagnosis of SIADH (hypoosmolar, euvolemia) will have some of the following features
- Low serum osm
- Inappropriately elevated urine osm (at least above 100)
- Urine sodium usually >40
- Exclude: thyroid, adrenal, diuretics as etiologies
- RCC: although our usual associate with RCC is polycythemia as a paraneoplastic effect, anemia is actually more commonly seen (thanks HH for the tip)!. According to this reference from Leslie, this is largely due to “nutritional status and the presence of a chronic disease”. Also some thoughts of an iron-binding protein that RCC produces.
- Diarrhea: think acute vs chronic
- Acute – usually infectious
- Chronic (>3 weeks) – inflammatory vs fatty vs watery (secretory vs osmotic vs functional). Calculate a stool osm gap to help with watery causes!