Can a Diuretic Cause Low Sodium? Thiazides, Blood Tests and Warning Signs

Yes. Diuretics, often called water pills, can lower the level of sodium in the blood because they increase the loss of sodium through the urine. MedlinePlus lists diuretic medicines among the recognised causes of low blood sodium (hyponatremia). A low result is a reason to talk to the prescriber, not a reason to stop the medicine on your own.

This guide is for adults, and the people who care for them, who have been told a sodium result is low while taking a diuretic. It focuses on thiazide-type diuretics. For what a low sodium result means in general and the wider list of causes, see our guide to the low sodium blood test. This guide cannot diagnose the cause of your result.

MedlinePlus describes a severe drop in sodium as a possible life-threatening emergency. Cleveland Clinic advises going to an emergency department for confusion, seizures, difficulty waking up, severe headache, or severe vomiting or diarrhoea. NHS advice for people taking bendroflumethiazide, a thiazide diuretic, says to call emergency services if someone suddenly becomes very confused, drowsy or dizzy, or faints and cannot be woken.

Seek medical advice the same day for new muscle weakness or cramps, nausea, unusual tiredness, unsteadiness or a fall, especially if a diuretic was started or changed recently. In older adults, changes in thinking or balance can be subtle and may be noticed first by family or carers.

• Emergency: new or sudden confusion, seizures, very drowsy or hard to wake, fainting without recovery, severe headache.
• Same-day advice: new unsteadiness or falls, muscle cramps or weakness, nausea or vomiting, signs of dehydration such as dark urine or passing less urine than usual.

Thiazide diuretics help the body pass out extra fluid in the urine, which lowers blood pressure and reduces swelling. NHS information on bendroflumethiazide notes that it can lower blood salt (electrolyte) levels. Hydrochlorothiazide is another commonly used water pill; its MedlinePlus information lists confusion and muscle cramps among symptoms to report right away.

A 2016 review in the Journal of Geriatric Cardiology describes thiazide-associated low sodium as more common in older people, women, people with low body weight, and people who follow a low-salt diet or drink large amounts of water. The same review reports that, in one group of outpatients taking thiazides, being over 70 was linked with a much higher risk. These are population patterns, not a prediction for any individual.

The 2016 review reports that thiazide-associated low sodium is usually seen within the first few weeks of treatment, but it can also appear after years, often when something else changes. Examples include a new illness with vomiting or diarrhoea, a new medicine, a change in diet or drinking much more fluid than usual.

This is why a sodium result taken during an illness may look different from earlier results. Bring dates of any recent illness, medicine changes and diet changes to the appointment.

NHS guidance for bendroflumethiazide says people should see their doctor or nurse regularly for blood pressure checks and blood tests, which check that the kidneys are working well and that blood salt levels are balanced. How often depends on why the medicine is taken, any recent dose change and other health problems. MedlinePlus advice for hydrochlorothiazide also mentions occasional blood tests.

The 2016 review suggests checking kidney function and electrolytes, including sodium, soon after a thiazide is started, and checking sodium promptly if symptoms such as falls or reduced attention appear. Your own monitoring plan is set by the prescriber.

Cleveland Clinic describes sodium below 135 mEq/L as lower than normal. Reference ranges and units differ between laboratories, so compare your result with the range printed on your own report. MedlinePlus notes that a result outside the range does not always mean a condition that needs treatment, and that sodium is read alongside the other electrolytes.

Do not stop or change a prescribed medicine, or its dose, without talking to the prescriber. MedlinePlus advises not to stop hydrochlorothiazide without talking to your doctor, and NHS information notes that blood pressure is likely to go back up, or fluid may return, if bendroflumethiazide is stopped. Some other medicines can be dangerous to stop suddenly, so bring a full list rather than pausing anything yourself.

Avoid making big diet or fluid changes on your own, too. NHS guidance advises avoiding very strict low-salt diets while taking bendroflumethiazide, but your clinician may have given you specific advice for heart, kidney or liver conditions. Ask before changing salt or fluid intake.

A diuretic may not be the only reason for low sodium. MedlinePlus lists other causes including diarrhoea, vomiting, heavy sweating, heart failure, kidney disease, advanced liver disease and a hormone condition called SIADH. Cleveland Clinic also names some antidepressants (SSRIs) as a cause. Clinicians may use blood and urine tests to look at the whole picture.

Bring the written result, the reference range, earlier results and a complete list of medicines, including non-prescription products and supplements.

• How low is my sodium compared with the lab's range, and how does it compare with my earlier results?
• Do you think my diuretic is contributing, or could something else be involved?
• Do I need a repeat blood test, and when?
• Should I change anything about my salt or fluid intake, or keep things as they are?
• Which symptoms mean I should seek urgent help before the next test?
• Do any of my other medicines also affect sodium?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a low sodium result on a diuretic described their tests, timeline and questions can help you notice what to ask at your own appointment. Other people's cases cannot tell you the cause of your result or what treatment you need; your own clinician makes those decisions.