When your blood sugar climbs too high, your body starts trying to flush the excess glucose out through your urine, triggering a chain of symptoms that can escalate from mild discomfort to a life-threatening emergency. A normal fasting blood sugar sits below 100 mg/dL, and anything consistently above 180 mg/dL after meals starts causing noticeable problems. What happens next depends on how high it goes, how long it stays elevated, and whether your body can still produce insulin.
Why High Blood Sugar Makes You Thirsty and Exhausted
The first thing most people notice is needing to urinate far more than usual, followed by intense thirst. This happens because your kidneys can only reabsorb so much glucose. When blood sugar exceeds their capacity, the extra glucose spills into your urine and drags water along with it. Your body is essentially losing fluid every time you go to the bathroom, which triggers the thirst signal to compensate. This cycle of urinating and drinking can leave you dehydrated even if you’re consuming plenty of fluids.
That fluid loss also pulls important electrolytes like sodium, potassium, and chloride out of your body. The combined effect is fatigue, muscle weakness, and a general feeling of being unwell. Meanwhile, if your cells can’t access the glucose in your blood (because insulin is absent or not working properly), they’re starved for energy despite there being plenty of sugar circulating. This is why people with dangerously high blood sugar often feel exhausted and weak even though they’ve been eating normally.
Blurred Vision From Blood Sugar Spikes
High blood sugar can change your eyesight within hours. The lens of your eye is sensitive to shifts in fluid balance, and when blood sugar rises, the osmotic pressure around the lens changes. Water moves in or out of the lens, physically altering its thickness and curvature. This warps how light focuses on your retina, causing blurry vision that can shift day to day. The effect is temporary and usually resolves once blood sugar stabilizes, but people sometimes mistake it for needing new glasses. If your prescription seems to change rapidly, unstable blood sugar is a common culprit.
When High Blood Sugar Becomes an Emergency
Two distinct emergencies can develop from severely elevated blood sugar, and both can be fatal without treatment.
Diabetic Ketoacidosis
Diabetic ketoacidosis, or DKA, happens when your body has so little usable insulin that cells can’t take in glucose at all. Your liver switches to breaking down fat for fuel, which produces acids called ketones. When ketones build up faster than your body can clear them, your blood becomes dangerously acidic. DKA is diagnosed when blood sugar exceeds 200 mg/dL alongside high ketone levels and acidic blood chemistry. It develops most often in people with type 1 diabetes but can occur in type 2 as well.
Early signs include extreme thirst and frequent urination. If untreated, symptoms escalate quickly to fast and deep breathing, nausea and vomiting, stomach pain, fruity-smelling breath, dry skin and mouth, and severe fatigue. The fruity breath is a distinctive warning sign caused by acetone, one of the ketones your body produces. If your blood sugar stays above 300 mg/dL, your breath smells fruity, or you can’t keep food or water down, that’s an emergency.
Hyperosmolar Hyperglycemic State
Hyperosmolar hyperglycemic state, or HHS, is less common but more deadly. It typically strikes people with type 2 diabetes and involves blood sugar levels above 600 mg/dL. At these levels, the blood becomes extremely concentrated and thick. Unlike DKA, the body still produces enough insulin to prevent major ketone buildup, so the warning signs are subtler. Severe dehydration, confusion, and eventually coma develop over days to weeks rather than hours. The mortality rate for HHS is roughly 10% to 20%, making it one of the most dangerous complications of uncontrolled diabetes.
How Chronic High Blood Sugar Damages Blood Vessels
The long-term damage from persistently elevated blood sugar happens at the molecular level, through a process involving compounds called advanced glycation end products. When excess glucose lingers in the bloodstream, it bonds to proteins throughout the body in a way that permanently changes their structure. These altered proteins accumulate on the walls of blood vessels, forming rigid cross-links on collagen and elastin, the proteins that normally keep vessel walls flexible. Over time, this stiffens arteries and capillaries.
The damage goes deeper than stiffness. These sugar-modified proteins also block nitric oxide, the molecule your blood vessels rely on to relax and dilate. Without adequate nitric oxide, vessels constrict, blood pressure rises, and the inner lining of blood vessels becomes more permeable, allowing fatty deposits to penetrate the vessel wall. This accelerates atherosclerosis. The same compounds also trigger ongoing inflammation by activating immune cells and increasing oxidative stress, creating a self-reinforcing cycle of vascular injury. This is why people with poorly controlled diabetes face dramatically higher rates of heart attack and stroke.
Kidney Damage Over Time
Your kidneys filter blood through millions of tiny blood vessels, and those vessels are especially vulnerable to the damage described above. Diabetic kidney disease develops silently, often producing no symptoms until significant damage has occurred. The earliest detectable sign is small amounts of protein leaking into the urine, a condition called microalbuminuria, defined as 30 to 300 milligrams of albumin in a 24-hour urine collection. Healthy kidneys keep protein in the blood, so finding it in urine means the filters are breaking down.
If blood sugar remains poorly controlled, that protein leak worsens to macroalbuminuria (above 300 milligrams), signaling more advanced damage. From there, kidney function can decline progressively toward kidney failure. The American Diabetes Association recommends annual screening for kidney damage starting at the time of a type 2 diabetes diagnosis or five years after a type 1 diagnosis. Catching it early, when only small amounts of protein are leaking, gives you the best chance of slowing or halting the decline with blood sugar and blood pressure management.
Nerve Damage Starts in the Feet
Peripheral neuropathy is the most common type of nerve damage from high blood sugar, and it follows a predictable pattern. It affects the feet and legs first, then gradually moves to the hands and arms. Symptoms include numbness, tingling, burning, or sharp pain, often worse at night. The onset is slow enough that many people don’t realize it’s happening until significant nerve damage has already occurred.
The real danger of neuropathy isn’t just discomfort. When you lose sensation in your feet, you can step on something sharp, develop a blister, or burn yourself without feeling it. Those unnoticed injuries become the entry point for infections, especially because high blood sugar also impairs your body’s ability to heal.
Slower Healing and Higher Infection Risk
High blood sugar disrupts wound healing on multiple fronts. Your immune cells, particularly macrophages (the cells responsible for cleaning up damaged tissue and fighting bacteria), become locked in an inflammatory state that prevents them from switching to their repair role. Instead of clearing debris and promoting new tissue growth, they keep pumping out inflammatory signals, stalling the healing process.
Collagen production also suffers. The cells that normally knit wounds together, called fibroblasts, struggle to transition into their wound-repair mode when blood sugar is chronically elevated. The sugar-modified proteins that accumulate in tissue directly interfere with collagen cross-linking, the process that gives new tissue its strength. The result is wounds that stay open longer, heal with weaker tissue, and are far more prone to infection. This is why diabetic foot ulcers are such a serious complication, particularly when combined with the numbness from neuropathy that prevents people from noticing the wound in the first place.
Blood Sugar Levels and What They Mean
Not every blood sugar spike is an emergency, but knowing the thresholds helps you gauge how urgently to act. A reading between 180 and 250 mg/dL is high enough to cause symptoms like thirst, frequent urination, and blurry vision. Sustained levels in this range drive the long-term damage to blood vessels, kidneys, nerves, and eyes described above.
Above 300 mg/dL, you’re in territory where DKA becomes a real risk, especially if you have type 1 diabetes or are ill. Above 600 mg/dL, HHS is a concern. At any level, the combination of high blood sugar with vomiting, confusion, fruity-smelling breath, or difficulty breathing signals a crisis that needs immediate medical attention. If you have diabetes and notice your readings creeping upward consistently, that trend matters just as much as any single spike, because the complications of chronic high blood sugar develop quietly over months and years before they become obvious.