SIADH (syndrome of inappropriate antidiuretic hormone)
A condition in which the body releases too much of the water-retaining hormone ADH, so the kidneys hold on to water and dilute the blood — causing a low sodium. It is a common cause of low sodium, often from medications, lung or brain problems, or cancer, and is worked out with blood and urine tests.
What it is
Antidiuretic hormone (ADH, also called vasopressin) tells the kidneys to hold on to water. In SIADH, ADH is released when it should not be, so the body retains water inappropriately. The retained water dilutes the blood, and the hallmark result is a low sodium (hyponatremia) with a low blood osmolality, while the urine is inappropriately concentrated because the kidneys keep holding water. Importantly, the person is not obviously dehydrated or fluid-overloaded — the body's water is simply too diluted. SIADH is one of the commonest causes of a low sodium found on blood tests. The triggers are many: certain medications (some antidepressants, antiepileptics, chemotherapy and others), lung conditions (pneumonia, and small-cell lung cancer, which can make ADH itself), brain and nervous-system problems (stroke, injury, infection, surgery), pain, nausea and major stress. Diagnosis is a matter of pattern — a low, dilute blood sodium with concentrated urine, and normal thyroid, adrenal and kidney function — after excluding other causes. Treatment addresses the trigger and usually involves careful fluid restriction; the correction of sodium must be done slowly, as too-rapid correction is harmful.
Key lab markers
- Sodium — low (hyponatremia); the defining abnormality.
- Serum osmolality — low (dilute blood).
- Urine osmolality — inappropriately high (concentrated urine despite dilute blood).
- Urine sodium — typically raised, helping distinguish SIADH from dehydration.
- Thyroid and adrenal function (TSH, cortisol) — checked to exclude other causes of low sodium.
Symptoms
- Often none if the sodium falls slowly
- Nausea, headache
- Difficulty concentrating, confusion
- Tiredness, weakness
- Muscle cramps
- Irritability, restlessness
- If sodium is very low or falls fast: drowsiness, seizures, reduced consciousness
Related lab panels
When to discuss with a doctor
SIADH is usually recognised when a blood test shows a low sodium and the doctor works through the cause. Mild, slowly developing lows may cause few symptoms but still need evaluation, especially to review any medication that might be responsible. A low sodium that is severe, or has fallen quickly — with headache, vomiting, marked confusion, drowsiness or seizures — is a medical emergency needing urgent care. The correction of a low sodium must be done carefully and gradually under medical supervision, because raising it too fast can cause serious harm, so this is not something to self-treat. Mediora.AI can highlight the low-sodium, low-osmolality pattern that points toward SIADH and prompt the right follow-up questions, but the diagnosis, cause-finding and treatment belong with your doctor.