SIAD (aka SIADH)

I’m going to take a stab at explaining a somewhat complex but clinically relevant topic: Hyponatremia. This matters for us because several psychiatric medications can cause SIAD (syndrome of inappropriate antidiuresis).

You may have learned it as SIADH. The preferred term now is SIAD, because not all cases involve elevated measurable ADH levels.

Psychiatric Medications That Can Cause SIAD

Most commonly associated:

• Oxcarbazepine
• Carbamazepine
• Serotonergic antidepressants (SSRI, SNRI, MAOI, TCA)

Less commonly but can also occur:

• Lamotrigine
• Valproate/divalproex
• First- and second-generation antipsychotics

Many non-psychiatric medications and numerous medical conditions can also cause SIAD.

• Treatment of SIAD can involve fluid restriction and/or re-eval of meds as well as other treatments not discussed here.

How to Evaluate Hyponatremia

The first step is to check serum osmolality.

Serum osmolality reflects how concentrated the blood is. 

There are three possibilities:

• Low serum osmolality → Hypotonic hyponatremia

• Normal serum osmolality → Isotonic (pseudohyponatremia)

• High serum osmolality → Hypertonic hyponatremia

The type we are concerned about in SIAD is:

👉 Low serum osmolality (hypotonic hyponatremia)

This means there is excess water relative to sodium in the extracellular compartment.

Next Step in Hypotonic Hyponatremia

Once hypotonic hyponatremia is confirmed, the next step is to check:

• Urine osmolality
• Urine sodium

Of these, urine osmolality is the key first test.

Urine Osmolality

In SIAD:

• Urine osmolality is elevated (>100 mOsm/kg)

Key concept:

High urine osmolality = concentrated urine = ADH is active.

ADH (antidiuretic hormone) conserves free water by making the kidney reabsorb water.
When ADH is on, urine becomes concentrated.

In contrast:

Primary polydipsia (the main psychiatric alternative diagnosis) shows:

• Low urine osmolality (<100 mOsm/kg)
• Because ADH is suppressed and the kidneys are appropriately dumping excess water.

So:

• High urine osm → ADH on
• Low urine osm → ADH off

Urine Sodium

Urine sodium is helpful but less definitive.

In classic SIAD:

• Urine sodium is typically >40 mEq/L

This happens because SIAD causes mild volume expansion from water retention.
That mild expansion suppresses RAAS, and the kidneys excrete sodium normally or even at higher levels (natriuresis).

Hypovolemia

Another important cause of hypotonic hyponatremia is hypovolemia.

When effective circulating volume is reduced, the body responds by activating:

• ADH → causing high urine osmolality (concentrated urine)
• RAAS → causing low urine sodium (the kidneys conserve sodium)

So the classic hypovolemic pattern is:

• Low serum osmolality
• High urine osmolality
• Low urine sodium

Reduced effective circulating volume can occur in two main ways:

1. True hypovolemia
   • Vomiting
   • Diarrhea
   • Poor intake
   • Diuretics
   • Third spacing (eg, pancreatitis)
2. Edematous states with reduced effective arterial blood volume
   • Heart failure
   • Cirrhosis

Richa Vijayvargiya, MD

Psychiatry Service Director, UF Shands

Associate Program Director, UF Psychiatry Residency

Assistant Clinical Professor

UF Department of Psychiatry

Consultation-Liaison Division

rvijayvargiya@ufl.edu