SIADH versus diabetes insipidus: one hormone, opposite failures
The one fact. Antidiuretic hormone has exactly one job worth memorizing. It opens water channels in the collecting duct so that water is returned to the blood instead of leaving in the urine. Too much hormone and water is trapped in the body. Too little and water pours out. Every other finding on this page follows from that single sentence.
| SIADH (too much hormone) | DIABETES INSIPIDUS (too little effect) | |
|---|---|---|
| Cause | Small cell lung cancer, head injury, central nervous system infection, and certain antidepressants and anticonvulsants | Central form after pituitary surgery or head trauma; nephrogenic form has hormone present but kidneys that will not answer it |
| Urine | Scant and concentrated, specific gravity high | Copious and nearly colorless, specific gravity at the bottom of the scale → BOOK 07 · P11 |
| Serum sodium | Diluted and falling | Concentrated and rising |
| Weight and volume | Weight gain without visible edema, because the retained water distributes into cells | Weight loss with hypovolemia, tachycardia, and intense thirst |
| What kills | Cerebral edema and seizure from hyponatremia | Hypovolemic collapse and hypernatremic neurologic injury |
| First nursing actions | Restrict fluid, assess neurologic status hourly, institute seizure precautions, weigh daily on the same scale | Replace fluid, record hourly intake and output, weigh daily, keep water within reach for the alert patient, anticipate desmopressin in the central form |
Watch outin severe symptomatic hyponatremia, hypertonic saline may be ordered, and the correction must be slow. Raising the serum sodium too quickly causes osmotic demyelination, a permanent neurologic injury. If your patient's sodium is climbing faster than the ordered rate, that is a call to the provider, not a success.
Margin notedesmopressin as a drug is → BOOK 05 · P9. Sodium and specific gravity reference values are → BOOK 07.
Memory hookSIADH SOAKS, DI DRAINS.
Margin notelook at the urine before you look at the sodium.
Takeawaythe urine tells you which condition it is, and the serum sodium then moves in the opposite direction.