Electrolytes and dehydration on GLP-1 therapy: sodium, potassium, bicarbonate, and the pattern the labels ask prescribers to catch

The electrolytes are the four or five small numbers at the top of the metabolic panel that most people skip past. On GLP-1 therapy they are the numbers that describe the one situation the labels single out for a lab check: a person who has been vomiting, has had diarrhea, or has simply stopped drinking because nothing appeals, and whose kidneys are running short of fluid.

What the labels say

The Wegovy label (section 5.5) reports postmarketing cases of acute kidney injury, some needing dialysis, and says most occurred in patients with gastrointestinal reactions leading to dehydration: nausea, vomiting, diarrhea. It instructs prescribers to monitor renal function in patients reporting adverse reactions that could lead to volume depletion, especially during dosage initiation and escalation. The Ozempic (5.6), Zepbound (5.3) and Mounjaro (5.5) labels say the same. "Renal function" in practice is a metabolic panel, and the electrolytes come with it.

The numbers and what moves them

Sodium (MedlinePlus: 135 to 145 mEq/L) tracks water balance more than salt intake. It rises when you lose water faster than salt (sweating, not drinking) and falls when you lose salt and replace with plain water, or when the body holds water. Both directions cause confusion, weakness and, at extremes, seizures.

Potassium (3.7 to 5.2 mEq/L) is lost in vomit and stool. Low potassium causes muscle weakness, cramps and heart rhythm disturbance. High potassium is the opposite problem and is a feature of failing kidneys and of some blood pressure drugs when the kidneys are stressed. MedlinePlus lists ACE inhibitors and ARBs among causes of a high result.

Chloride (98 to 107 mEq/L) moves with sodium and bicarbonate. It falls with prolonged vomiting (stomach acid is hydrochloric acid).

Bicarbonate, reported as CO2 (23 to 29 mEq/L), is the body's main buffer. It falls in diarrhea and in ketosis from not eating; it rises with prolonged vomiting. On a panel it is the number that says whether the fluid problem has become an acid-base problem.

Magnesium (1.7 to 2.2 mg/dL) is not on the standard metabolic panel and is ordered separately. It falls with diarrhea and with long-term acid-reducer use, and a low magnesium makes a low potassium hard to correct.

The dehydration pattern

On a single panel, dehydration looks like this: BUN up more than creatinine, creatinine up from your baseline, sodium normal or high, bicarbonate low if diarrhea or not eating, high if mainly vomiting, and hematocrit up on the CBC because the plasma has shrunk. No single value makes the diagnosis; the combination, plus the history of a bad week, does. This is why a baseline panel from before therapy matters: a creatinine of 1.1 means one thing if your baseline was 1.0 and another if it was 0.7.

Who is more exposed

The labels' warning applies to everyone, but three groups of medicines turn a rough week into a kidney problem faster.

• Diuretics keep pulling fluid out regardless of intake.
• ACE inhibitors and ARBs block the kidney's compensating mechanism when blood volume falls. MedlinePlus lists both as causes of high potassium.
• SGLT2 inhibitors increase urine output and can cause ketoacidosis when carbohydrate intake stops. Metformin accumulates if filtration falls.

People with existing kidney disease start with less reserve. People over 65 feel thirst less reliably. The starting and escalation weeks, when gastrointestinal reactions peak, are the labels' named window.

How often

• Baseline: on the initial metabolic panel.
• Trigger: vomiting or diarrhea for more than a day, inability to keep fluids down, dizziness on standing, very dark or scant urine, confusion. This is the labels' instruction: check renal function, which brings the electrolytes with it.
• Otherwise: with the annual panel. There is no schedule for electrolytes in a well person.

What to discuss with your prescriber

• A sick-day plan: which medicines to pause during a fluid-losing illness, and when to restart them.
• What symptoms should prompt a same-day call and a panel.
• If you are on a diuretic, ACE inhibitor, ARB or SGLT2 inhibitor: whether the escalation weeks need closer attention.
• What your baseline creatinine and sodium were, so you can recognise a change.

The dose-by-dose gastrointestinal reaction rates that set the odds of a rough week are on the Tirzepatide Hub and Semaglutide Hub. The kidney function page covers what happens to creatinine and eGFR once fluid is lost.