A Low Potassium with Normal Kidney Function
Most low potassiums come from losing it, through the gut or through the urine, and diuretics and diarrhea cover the bulk of them. The one thing to take away is that potassium will not correct while magnesium is low, no matter how much potassium you take. Magnesium is left off most panels, so it is the reason a stubborn low potassium stays stubborn.
The pattern on your report
- Potassium Low · mild Key
- Creatinine Normal Key
- Bicarbonate Normal Key
- Magnesium Low-normal Supporting
Printed as: Bicarbonate in mmol/Lor mEq/L— Labeled bicarbonate, HCO3 or total CO2 depending on the report.Creatinine in umol/Lor mg/dLMagnesium in mmol/Lor mg/dL— Genuinely different figures: 0.8 mmol/L is about 1.9 mg/dL.Potassium in mmol/Lor mEq/L— The same figure under both names.
Why the numbers look like this
Almost all your potassium is inside cells, and the small amount in the blood is what gets measured. That means the number can fall two different ways: you can genuinely lose potassium from the body, or the potassium you have can shift into cells and out of the blood.
Losses go through the gut or the kidney. Diarrhea, vomiting and laxatives take the gut route; diuretics and excess aldosterone take the kidney route, and the bicarbonate on the same panel tells you which. Shifts happen with insulin, with salbutamol, and with the alkalosis that vomiting produces, and there the total body potassium is normal.
Magnesium sits underneath all of it. Without enough magnesium the kidney leaks potassium continuously, so replacement fails until the magnesium is corrected.
Not being flagged is not the same as normal
The printed floor sits near 3.5 mmol/L. Real risk lives well below that line, and it depends heavily on context: the same potassium carries a different weight in someone taking digoxin or with a heart condition than in an otherwise healthy person. A single mildly low value in a well person after a stomach upset is a different object from a persistent one in someone on a diuretic, and the report does not distinguish them.
What else on the report can hide this
Bicarbonate is the number that splits the causes cleanly. A low potassium with a raised bicarbonate means an alkalosis, which points at vomiting, diuretics, or excess aldosterone. A low potassium with a low bicarbonate points instead at diarrhea or a renal tubular acidosis, which is an entirely different investigation.
Blood pressure completes the picture and is not on the panel at all. A persistently low potassium with high blood pressure and a raised bicarbonate is the classic primary hyperaldosteronism presentation, which is treatable, substantially under-diagnosed, and it matters because it changes how the blood pressure itself is managed. Add magnesium to whatever else gets requested, since without it the rest of the work is wasted.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Diuretics
Thiazides and loop diuretics both waste potassium in the urine. Usually with a raised bicarbonate, and the commonest cause in anyone treated for blood pressure or heart failure.
- Very common
Gut losses
Diarrhea, vomiting, laxative use, or a stoma. Diarrhea lowers bicarbonate while vomiting raises it, so the same symptom group splits on that one number.
- Common
Low magnesium
Causes continuous renal potassium wasting and makes replacement fail. Common with proton pump inhibitors, alcohol and diuretics.
- Common
Potassium shifting into cells
Insulin, albuterol inhalers or nebulizers, the treatment of high blood sugar, and refeeding after a period of poor intake. Total body potassium is normal and the level recovers on its own.
- Common
Poor intake with another loss
Rarely enough on its own, but it turns a modest loss into a low result. Alcohol use and eating disorders both contribute.
- Uncommon
Primary hyperaldosteronism
Low potassium with high blood pressure and a raised bicarbonate. Treatable, and a recognized cause of blood pressure that has not responded to several drugs.
- Uncommon
Licorice, or a high-dose steroid course
Glycyrrhizin in licorice and some herbal products mimics aldosterone, and high-dose steroids do similar. Low potassium with a raised bicarbonate and often a raised blood pressure, and it reverses on stopping.
- Uncommon
Renal tubular acidosis
Low potassium with a low bicarbonate and a normal anion gap. The bicarbonate direction is what separates it from everything above.
- Rare
Rare inherited tubular disorders, or periodic paralysis
Gitelman and Bartter syndromes, suspected in a young person with a persistent unexplained loss, normal blood pressure and low magnesium. Periodic paralysis, familial or with an overactive thyroid, shifts potassium into cells suddenly with weakness.
What is usually checked next
- Magnesium Determines whether replacement can work at all. The most useful test on this list.
- Bicarbonate with a chloride Splits the causes into the alkalosis group and the acidosis group, which have almost no overlap.
- Blood pressure, measured properly Low potassium with hypertension points at aldosterone, and it needs no blood test.
- Urine potassium Says whether the loss is through the kidney or the gut, which the history does not always make obvious.
- Aldosterone and renin Tests for hyperaldosteronism, and is worth doing before spironolactone is started because the drug interferes with the result.
When to seek care sooner
- Emergency Palpitations, fainting, or an irregular heartbeat
- Emergency Severe muscle weakness, or difficulty breathing
- Emergency A potassium the laboratory reports as critically low
- Same day A low potassium in anyone taking digoxin
- Same day Persistent vomiting or diarrhea that will not settle
- Soon A low potassium that does not correct despite supplements
Questions worth bringing to your appointment
- Has my magnesium been checked?
- Could my diuretic be responsible, and is there an alternative?
- What is my bicarbonate doing, and what does that suggest?
- Given my blood pressure, is hyperaldosteronism worth testing for?
- How soon should this be rechecked after starting replacement?