The main cause of high potassium is kidney disease. Your kidneys are responsible for filtering excess potassium out of your blood, and when they lose that ability, potassium builds up. Normal blood potassium falls between 3.5 and 5.0 mmol/L, and levels at or above 5.5 mmol/L are considered high (hyperkalemia).
Why Kidney Disease Is the Leading Cause
About 90% of the potassium you consume leaves your body through urine. When kidney function declines, as it does in chronic kidney disease (CKD), the filtration rate drops and potassium that would normally be excreted stays in the bloodstream. The more advanced the kidney disease, the harder it becomes for the body to maintain safe potassium levels.
What makes this worse is that many people with kidney disease take medications designed to protect their kidneys and heart. These drugs, commonly prescribed for high blood pressure and heart failure, work by blocking a hormonal system that regulates sodium and potassium balance. The trade-off is that while they slow kidney damage and improve heart outcomes, they also reduce the kidneys’ ability to excrete potassium. The combination of impaired kidney function and these medications is the single most common setup for dangerously high potassium.
Medications That Raise Potassium
Even in people with relatively healthy kidneys, certain medications can tip potassium levels higher. Blood pressure drugs that block the renin-angiotensin system (ACE inhibitors and ARBs) are the most well-known culprits, but they’re not the only ones. Common anti-inflammatory painkillers (NSAIDs like ibuprofen and naproxen) increase the odds of high potassium by roughly 50% in older adults, particularly those who already have mild kidney impairment. The risk climbs when these drugs are combined with blood pressure medications or used alongside potassium-sparing diuretics.
Heparin, a blood-thinning medication used in hospitals, can also raise potassium by blocking the production of aldosterone, a hormone your adrenal glands make specifically to help the kidneys shed potassium.
How Hormones Control Potassium
Your body uses two main hormones to keep potassium in check: aldosterone and insulin. Aldosterone tells the kidneys to excrete potassium. Insulin helps shuttle potassium from the bloodstream into cells, where it belongs. If either system fails, potassium rises.
In Addison’s disease, the adrenal glands don’t produce enough aldosterone. This leads to potassium retention, though it’s typically mild to moderate because insulin and other backup mechanisms partially compensate. The opposite condition, Conn’s syndrome, involves too much aldosterone and causes potassium to drop abnormally low. These hormonal disorders illustrate how tightly the body normally regulates this mineral, and how disruption at the hormonal level can shift the balance.
Cell Damage and Potassium Release
Most of the potassium in your body, roughly 98%, sits inside your cells. When large numbers of cells are destroyed at once, all that stored potassium floods into the bloodstream faster than the kidneys can clear it. This happens in a few specific situations: severe muscle injury (rhabdomyolysis) from crush injuries, extreme exercise, or prolonged immobilization; massive burns; and tumor lysis syndrome, where cancer treatment kills a large volume of tumor cells simultaneously. In these cases, the potassium spike can be sudden and severe.
Acidosis and Potassium Shifts
When your blood becomes too acidic, a condition called metabolic acidosis, potassium moves out of cells and into the bloodstream even without any cell damage. This happens because the body tries to buffer excess acid by swapping hydrogen ions into cells and pushing potassium ions out. At the same time, acidosis impairs the kidneys’ ability to excrete potassium and reduces the activity of the pump that normally pulls potassium back into cells. The result is a triple hit: more potassium leaving cells, less going back in, and less being filtered out. This is common in uncontrolled diabetes, severe dehydration, and kidney failure.
Diet and Salt Substitutes
For most people with healthy kidneys, eating potassium-rich foods like bananas, potatoes, and spinach won’t cause problems. The kidneys simply adjust and excrete more. The risk changes, however, when kidney function is compromised.
Salt substitutes deserve special attention. Many brands replace sodium chloride with potassium chloride, sometimes making up 25% of the product. A large clinical trial found that using these potassium-enriched salt substitutes increased the risk of biochemical hyperkalemia, particularly among people with impaired kidneys or those taking blood pressure medications that already reduce potassium excretion. If you have kidney disease, these products can quietly push your levels into a dangerous range with everyday cooking.
False-High Readings
Not every high potassium result on a blood test reflects what’s actually happening in your body. A phenomenon called pseudohyperkalemia occurs when blood cells break open during or after a blood draw, releasing their internal potassium into the sample. This is surprisingly common, contributing to roughly 40% of all laboratory errors and affecting up to 12% of samples in busy settings like emergency departments. Difficult blood draws, small needles, vigorous shaking of the tube, or samples sitting too long before processing can all cause it. The average false elevation is about 1.9 mmol/L, which is enough to make a normal result look dangerously high. When a potassium reading comes back elevated in someone with no symptoms and no obvious risk factors, a repeat draw is standard practice.
What High Potassium Does to the Heart
The reason high potassium matters so urgently is its effect on the heart’s electrical system. Excess potassium in the blood reduces the excitability of heart muscle cells, disrupting the signals that coordinate each heartbeat. Changes on an EKG typically don’t appear until levels reach about 6.0 mmol/L, but from that point, the progression can be rapid.
The earliest sign is tall, peaked T waves on the EKG tracing. As levels climb, the electrical signal slows: the P wave flattens, the interval between heartbeats lengthens, and the main electrical spike (QRS complex) widens. Above 9.0 mmol/L, the EKG can take on a sinusoidal wave pattern, which is a pre-terminal rhythm that can deteriorate into cardiac arrest. This is why potassium above 6.5 mmol/L is considered a critical value requiring immediate treatment.
Symptoms you might notice before reaching that point include muscle weakness, tingling or numbness, nausea, and a feeling that your heart is beating irregularly. Some people feel nothing at all, which is part of what makes chronic, slowly rising potassium so dangerous.