A Case of Hyponatremia

It is cool to see the in Plasma Sodium be close to the predicted value based on our calculation

Let’s review the case

An elderly man with metastatic cancer presents with Hyponatremia
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Hyponatremia work-up is consistent with SIADH due to the malignancy

Plasma Osmolality
Euvolemic
Urine Osmolality
Urine Na
Normal Uric Acid, TSH, Cortisol
On no medications that ADH
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Day # 1
Plasma Na =124 mEq/L
Urine Output/24 hrs = 1 L
Urine Osm. = 604 mOsm/kg
Wt = 59 kg
TBW = 35 L (0.6 x 59 kg)

Urine Osmolality x Urine Volume = Urine Solute Excretion
604 mOsm/L x 1L= 604
Pt’s daily solute excretion is 604 mOsm
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Pt. is started on urea 15 grams po bid = 30 grams/day
30 grams of urea = 500 mOsm

(In comparison:
6 grams of salt tabs = 205 mOsm)

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If you add 500 mOsm of urea to the patient’s daily solute excretion of 604 mOsm, then the daily solute excretion will to 1104 mOsm

Daily solute excretion =
604 + 500 = 1104 mOsm
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Urine Osmolality is fixed in SIADH so expected in Urine Output after starting urea 30 gram/day would be:

Daily Urine Solute Excretion/ Urine Osm. = Urine Volume per day

Let’s plug in the numbers:

1104 mOsm/ 604 = 1.8 L
UO on Day # 2 is 1.8L
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Now let’s calculate the expected rise in patient’s plasma Na with the in Urine Output due to urea administration

Edelman Equation:
Plasma Na = Na(e) + K(e)/TBW
-Na(e) is exchangeable Na
-K(e) is exchangeable K
-TBW is Total Body Water

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Let’s plug in the values from
Day # 1 into the equation first:

Day # 1
Plasma Na = Na(e) + K(e)/TBW
124 = Na(e) / 35
Na(e) = 124 x 35 = 4340

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On Day # 2 the UO es to 1.8L
So what would be the change in TBW?

35L plus water intake (1.5L in this patient) minus 1.8L (urine output) minus 0.8 L (insensible water loss)

So the new TBW = 33.9 L

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So what would be expected in Plasma Na on Day # 2

Urea does change Na intake so exchangeable Na is the same = 4340 But as shown above the TBW has changed due to in Urine Output

Day # 2
Plasma Na = 4340/ new TBW
= 4340/33.9 = 128 mEq/L

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Our patient’s Plasma Na was
127 mEq/L on Day # 2 - Labs

This case demonstrates how you can predict the in plasma Na in SIADH with urea administration

The patient tolerated oral urea well but sometimes it can be a limiting factor

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Here is a reference for the use of urea in patients with SIADH due to an underlying malignancy

https://pubmed.ncbi.nlm.nih.gov/30868608/?i=1&from=urea%20AND%20sIAD

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