Almost all of the hyponatremia I see is inpatient, but this week a woman was referred to my clinic with a sodium of 128. She has a sharp family doctor who ordered all the right tests. Here are the key pieces:
- Plasma sodium 128
- Plasma osmolality 277
- Urine osmolality 180
- Urine SpGrav 1.005
- Urine sodium 14
She has a history of hypothyroidism but her TSH was over suppressed, no hint of hypothyroidism. She was not taking any diuretics. She was on an SSRI that could cause SIADH but the low urine osmolality and low specific gravity argue against excessive ADH activity.
The case hinges on the low urine osmolality. This is a rare case of ADH-independent hyponatremia. All of the major causes of hyponatremia (volume depletion, diuretics*, heart failure, SIADH, etc) are driven by ADH which prevents the kidney from clearing free water. The low urine osmolality indicates the kidney is not under the influence of ADH and doing what it needs to in order to correct the sodium, i.e. excreting excess water in the form of dilute urine.
I believe there are only a few causes of ADH-independent hyponatremia, and only two occur with any regularity:
- Psychogenic polydipsia
- Tea and toast syndrome
- Reset osmostat (rare)
When the kidney is making dilute urine and the patient has hyponatremia the problem is not in the quality of the urine, which is appropriate, but in the quantity of urine, which is inadequate.
She described her diet as a peanut butter and jelly sandwich for breakfast, some fruit and juice with some nuts for lunch and nothing for dinner. Her fluid intake was high (4-5 cups of coffee, a large water bottle of water, some juice, some soda, and a couple of additional glasses of water) but no where near enough for psychogenic polydipsia.
I suspect she has tea and toast syndrome. I am checking a 24-hour urine osmolality to gauge her daily osmolar load and then plan to have her increase the amount of protein and minerals in her diet while trying to taper her fluid intake.
More on tea and toast syndrome here
* if the diuretics are still active the urine osmolality could be low but once the drug wears off the urine osmolality will climb
why do you say the U Osm is low? If there truly was no ADH, the U Osm would be < 100. U Osm of 180 is 'inappropriately' elevated.
I say the urine osmolality is low because it is less than the serum osmolality. A urine osmolality that is less than the serum osmolality will clear solute free water.
Even tiny amounts of ADH will increase the urine osmolality to isotonicity. This urine is 100 mOsm/Kg H2O below serum osmolality