How to diagnose SIADH

Several psychotropic medications can cause the syndrome of inappropriate anti-diuretic hormone secretion (SIADH). Most important among these are:

– Oxcarbazepine
– Carbamazepine
– Serotonergic antidepressants (SSRIs, SNRIs, MAOIs, TCAs)

Other psychotropic medications that may sometimes cause SIADH include lamotrigine, divalproex, and both first-generation and second-generation antipsychotics. Other than psychotropic medications, many non-psychotropic medications and many medical conditions can also cause SIADH.

SIADH presents with low serum sodium (hyponatremia) and non-specific clinical symptoms like fatigue, malaise, etc.

If a patient has hyponatremia, how should the diagnosis of SIADH be confirmed?

First, check if the person is dehydrated (dry mucous membranes, decreased skin turgor, tachycardia, orthostatic hypotension, etc.).

Next, make sure the person is not, the opposite, hypervolemic (edema, ascites).

Now, order these tests:

1. Serum creatinine--to rule out renal failure
2. Serum thyroid stimulating hormone (TSH)–to rule out severe hypothyroidism as a cause of low serum osmolality
2. Serum osmolality
3. Urine osmolality
4. Urinary sodium concentration

What will we find in a person with SIADH?

The person is retaining water so the serum osmolality will be low, typically less than 275 mOsml/Kg.

On the other hand, the urine will be unexpectedly concentrated. Urine osmolality will typically be more than 100 mOsm/Kg.

The urinary sodium concentration in SIADH is increased to more than 20 or 30 mmol/L while the patient is on normal salt and water intake (Verbalis et al., 2013).

What will we find in a person with polydipsia?

If the person is drinking excessive amounts of fluids, e.g., in psychogenic polydipsia, the urine will be dilute. That is, urine osmolality will also be low–less than 100 mOsm/Kg–and the urinary sodium concentration will also be low–less than 20 mmol/L.

Related Pages

Hyponatremia due to psychotropic medications

References

Grant P, Ayuk J, Bouloux PM, Cohen M, Cranston I, Murray RD, Rees A, Thatcher N, Grossman A. The diagnosis and management of inpatient hyponatraemia and SIADH. Eur J Clin Invest. 2015 Aug;45(8):888-94. PubMed PMID: 25995119; PubMed Central PMCID: PMC4744950.

Verbalis JG, Goldsmith SR, Greenberg A, Korzelius C, Schrier RW, Sterns RH, Thompson CJ. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013 Oct;126(10 Suppl 1):S1-42. doi: 10.1016/j.amjmed.2013.07.006. PubMed PMID: 24074529.

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