When your blood sugar climbs too high, your body sends clear warning signals: excessive thirst, frequent urination, and blurry vision. These are the earliest signs of hyperglycemia, and they happen because excess glucose in your bloodstream pulls water out of your tissues and overwhelms your kidneys. Left unchecked, high blood sugar can escalate into a medical emergency within hours or quietly damage your organs over months and years.
The First Symptoms You’ll Notice
The classic triad of high blood sugar is hard to miss once you know what to look for. You urinate more often because your kidneys are working overtime to flush out the excess glucose. As they do, they pull extra water along with it, a process called osmotic diuresis. That fluid loss makes you intensely thirsty, which leads to drinking more, which leads to urinating even more. It’s a cycle that can leave you dehydrated surprisingly fast.
Blurry vision is another early sign. High glucose levels change the fluid balance inside your eye lenses, temporarily warping their shape. You may also feel unusually tired, weak, or hungry despite eating normally. Some people notice dry skin, headaches, or difficulty concentrating. These symptoms can come on gradually, especially in type 2 diabetes, which is why they’re easy to dismiss as stress or poor sleep.
What’s Happening Inside Your Body
Normally, your body uses insulin to move glucose from your blood into your cells for energy. When that system breaks down, either because you don’t produce enough insulin or your cells resist it, glucose builds up in your bloodstream with nowhere to go. Your kidneys try to compensate by filtering out the excess, but they can only handle so much. Once blood sugar crosses a certain threshold, glucose spills into your urine and drags water with it.
This dehydration triggers a cascade. Your blood becomes more concentrated, your blood pressure can drop, and your electrolytes shift out of balance. Meanwhile, your cells are essentially starving for energy despite being surrounded by glucose they can’t absorb. In response, your body may start breaking down fat for fuel, which produces acidic byproducts called ketones.
When High Blood Sugar Becomes an Emergency
Two life-threatening conditions can develop when blood sugar stays very high without treatment: diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS).
Diabetic Ketoacidosis
DKA happens when your body produces dangerously high levels of ketones. It’s most common in type 1 diabetes but can occur in type 2 as well. Blood sugar typically rises above 250 mg/dL, and the flood of ketones makes your blood acidic. Early signs include nausea, vomiting, abdominal pain, and a fruity smell on your breath. As it worsens, you may experience labored breathing, confusion, and extreme fatigue. Without treatment, DKA can lead to coma.
If your blood sugar reads 240 mg/dL or higher, testing your urine for ketones with an over-the-counter kit can tell you whether your body has started down this path. A positive ketone test means you need medical attention promptly.
Hyperosmolar Hyperglycemic State
HHS is rarer but even more dangerous, with blood sugar levels soaring to 600 mg/dL or above. It develops more slowly than DKA, usually over days or weeks, and is more common in older adults with type 2 diabetes. The hallmark is extreme dehydration. Your blood becomes so concentrated that it pulls water from your organs, including your brain. Confusion, seizures, and loss of consciousness can follow. HHS carries a high mortality rate and requires emergency hospitalization.
Nerve Damage From Chronic High Sugar
When blood sugar stays elevated over months or years, the damage shifts from acute crises to slow, progressive injury. Nerve damage, called diabetic neuropathy, is one of the most common consequences. It usually starts in the feet and can spread to the legs, hands, and arms. You might feel tingling, numbness, burning, or sharp pain. Some people lose sensation entirely, which is dangerous because you may not notice cuts or injuries that can become infected.
Not all nerve damage affects your extremities. High blood sugar can also harm the nerves that control your internal organs. This can lead to digestive problems like nausea or feeling full after a few bites, bladder issues, sexual dysfunction, or abnormal heart rate. Some people develop damage to a single nerve, causing sudden weakness or pain in one specific area, often the face, torso, or a limb.
How Your Eyes Are Affected
Persistent high blood sugar damages the tiny blood vessels in your retina, the light-sensitive tissue at the back of your eye. In the earlier stage, those vessels leak fluid or swell, distorting your vision. Your body tries to repair the damage by closing off the weakened vessels, but this can backfire. In the more advanced stage, your retina grows fragile new blood vessels on its surface to compensate for the ones it lost. These replacement vessels are weak and prone to bleeding. They can cause retinal detachments and leaking into the gel-like fluid inside your eye.
If enough of the light-detecting cells in your retina stop working, the result is permanent vision loss. Diabetic retinopathy is a leading cause of blindness, but it progresses slowly enough that regular eye exams can catch it early, when treatment is most effective.
Kidney Damage Often Starts Silently
Your kidneys contain millions of tiny filtering units. Over time, high blood sugar causes these filters to thicken and scar. The earliest sign is small amounts of a protein called albumin leaking into your urine, something you wouldn’t notice without a lab test. This kidney damage can begin 5 to 10 years before any symptoms appear, which is why annual urine and blood tests are standard for people with diabetes.
As the damage progresses, your kidneys lose their ability to filter waste efficiently. Fluid retention, swelling in your legs, and rising blood pressure are later signs. In severe cases, the kidneys fail entirely, requiring dialysis or a transplant.
The Effect on Your Heart and Blood Vessels
High blood sugar quietly damages the lining of your blood vessels in ways that dramatically raise your risk of heart attack and stroke. Here’s the short version: excess glucose triggers a chain reaction that produces harmful molecules called free radicals. These molecules disable your blood vessels’ natural ability to stay relaxed and flexible. The vessel walls become inflamed, sticky, and stiff.
Once that inflammation starts, immune cells migrate into the vessel walls and begin forming fatty deposits, or plaques. In people with chronically high blood sugar, these plaques tend to be less stable, meaning they’re more likely to rupture. A ruptured plaque triggers a blood clot, which can block an artery feeding your heart or brain. To make matters worse, high blood sugar also makes platelets (the cells responsible for clotting) stickier than normal, so clots form more readily.
This is why cardiovascular disease is the leading cause of death in people with diabetes. The damage accumulates gradually and often produces no symptoms until a major event occurs.
Blood Sugar Levels That Signal Trouble
For context, a normal fasting blood sugar is below 100 mg/dL. Readings between 100 and 125 mg/dL fall in the prediabetes range. A fasting reading of 126 mg/dL or higher on two separate occasions indicates diabetes. After eating, blood sugar naturally rises, but it shouldn’t stay above 180 mg/dL for long in a healthy person.
The thresholds that matter most for emergencies: anything above 240 mg/dL warrants checking for ketones. Above 300 mg/dL with symptoms like vomiting, confusion, or heavy breathing means you should seek medical care immediately. Readings above 600 mg/dL put you in the range where hyperosmolar hyperglycemic state becomes a serious risk, and that is a true medical emergency.