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Comprehensive Notes

1. Core Concept

Both conditions revolve around one hormone: ADH, the body’s water-saving signal.

SIADH: Too much ADH → body holds water (soaked inside)

DI: Not enough ADH or kidneys ignore ADH → body loses water (dry inside)

The blood and urine move in opposite directions in each disorder.

1. SIADH — “Soaked Inside, All Diluted”
   What Happens

ADH is high → kidneys save water

Blood becomes diluted

Urine becomes concentrated

Classic Causes

Small cell lung cancer (ectopic ADH)

Head trauma

Pituitary surgery

SSRIs

Carbamazepine, vincristine

Severe pneumonia, meningitis

Severe pain or nausea

Hallmark Labs

Low sodium

Low serum osmo

High urine specific gravity

High urine osmo

Typical Patient Picture

Confusion, headache, lethargy

Weight gain (one kilogram equals one liter held)

High blood pressure

Puffy face or eyes

Not thirsty

Very low urine output, dark concentrated urine

Priority Interventions

Strict fluid restriction

Daily weights

Neuro checks every few hours

Seizure precautions (especially when sodium drops below one twenty)

Critical Medication

Hypertonic saline (three percent) for seizures or very low sodium

Must use a central line

Must correct sodium slowly (no more than eight to twelve points in twenty-four hours)

Major Warning

Correcting sodium too fast risks central pontine myelinolysis, an irreversible brainstem injury.

Never Do

Never give hypotonic fluids

Never give normal saline

Never increase free water

1. Diabetes Insipidus — “Dry Inside, All High”
   What Happens

Little or no ADH signal

Kidneys dump water

Blood becomes concentrated

Urine becomes extremely dilute

Two Types

Central DI

Pituitary does not make ADH

Causes: head trauma, brain tumors, pituitary surgery

Nephrogenic DI

Kidneys ignore ADH

Causes: lithium, some antibiotics, chronic high calcium

Hallmark Labs

High sodium

High serum osmo

Very low urine osmo

Very low specific gravity

Typical Patient Picture

Intense thirst

Clear water-like urine

Ten to twenty liters of urine per day

Rapid weight loss

Tachycardia, low blood pressure

Signs of hypovolemic shock

Priority Interventions

Aggressive fluid replacement (D5W or free water)

Hourly intake and output

Daily weights

Watch closely for shock

Stopping the Water Loss

Central DI: Give desmopressin (DDAVP)

Nephrogenic DI:

Stop lithium or offending drug

Give a thiazide diuretic (paradox: triggers earlier sodium and water reabsorption)

Major Warning

Never fluid restrict DI — causes immediate circulatory collapse.

1. SIADH vs DI: The Instant EN-KLEX Pattern
   Think Like a Nurse Bow-Tie Pattern

Low sodium + high urine osmo → SIADH

Action: fluid restrict

Safety: neuro checks, seizure precautions

High sodium + low urine osmo → DI

Action: free water, D5W, desmopressin

Safety: hourly intake and output, watch for shock

1. Bedside Pearl

If a post-pituitary surgery patient suddenly puts out large amounts of clear urine and their sodium is rising past one forty-five:
→ Stop what you’re doing and call the provider immediately.
This is a DI crisis until proven otherwise.

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