When your blood sugar is high, your body can’t move glucose from your bloodstream into your cells efficiently, so it starts trying to flush the excess out through your kidneys. A fasting blood sugar above 100 mg/dL is considered elevated, and levels above 126 mg/dL on two separate tests indicate diabetes. What you feel and what happens inside your body depends on how high your levels go and how long they stay there.
Why High Blood Sugar Causes Symptoms
Normally, insulin acts like a key that lets glucose pass from your blood into your cells for energy. When blood sugar is high, either because your body doesn’t produce enough insulin or your cells have stopped responding to it properly, glucose builds up in your bloodstream instead. Your cells are essentially starved for fuel even though your blood is saturated with sugar.
When blood glucose exceeds roughly 180 mg/dL, your kidneys hit their limit. They can no longer reabsorb all the glucose passing through, so it spills into your urine. That excess sugar pulls water along with it, a process called osmotic diuresis, which is why frequent urination is one of the earliest and most noticeable signs. The fluid loss triggers thirst, and drinking more fluids only feeds the cycle of urination. This loop of peeing more, getting dehydrated, and feeling increasingly thirsty is the hallmark pattern of uncontrolled blood sugar.
What It Feels Like Day to Day
The short-term symptoms of high blood sugar tend to build gradually, which makes them easy to dismiss or attribute to something else. Beyond thirst and frequent urination, you may notice:
- Fatigue and weakness, because your cells aren’t getting the glucose they need for energy
- Blurry vision, caused by fluid shifts in the lens of your eye
- Headaches, often from dehydration
- Unintentional weight loss, particularly in type 1 diabetes, as your body breaks down fat and muscle for fuel
- Slow-healing cuts or frequent infections, since high glucose impairs immune function
These symptoms can appear mild at first but tend to worsen the longer blood sugar remains elevated. Many people with type 2 diabetes live with moderately high blood sugar for months or years before they’re diagnosed, simply because the symptoms creep in slowly.
How High Blood Sugar Damages Blood Vessels
The long-term consequences of high blood sugar come down to what excess glucose does to the lining of your blood vessels. High glucose concentrations trigger a chain reaction inside vessel walls: cells produce more harmful molecules called free radicals, which overwhelm the protective molecule (nitric oxide) that normally keeps blood vessels relaxed and flexible. In people with diabetes, the enzyme responsible for making nitric oxide can malfunction so badly that it starts producing more free radicals instead. Research published in Circulation Research found that one key component of this damage pathway was increased sevenfold in diabetic tissue compared to healthy controls.
The result is blood vessels that are stiffer, more inflamed, and more prone to plaque buildup. This is why chronically high blood sugar raises the risk of heart attack, stroke, and poor circulation, particularly in smaller vessels that feed the eyes, kidneys, nerves, and feet.
Effects on Your Eyes, Kidneys, and Nerves
The smallest blood vessels in your body are the most vulnerable. In your eyes, high blood sugar weakens the tiny vessels in the retina, causing them to bulge into small pouches that can leak fluid. This is diabetic retinopathy. In its early stage, that leaking causes the central part of the retina (the macula) to swell, distorting your vision. Left unmanaged, the damage progresses and can eventually cause blindness. According to the CDC, this remains one of the leading causes of vision loss in adults.
Your kidneys filter roughly 50 gallons of blood every day, and the filtering units are made of delicate capillaries. Years of elevated blood sugar, combined with high blood pressure, gradually scars these filters and reduces their ability to clean your blood. The process is slow. Most people won’t notice kidney damage until it’s fairly advanced, which is why regular urine and blood tests matter if you have diabetes or prediabetes.
Nerves take a double hit. High glucose directly causes chemical changes that impair a nerve’s ability to transmit signals, and it also damages the small blood vessels that supply oxygen and nutrients to those nerves. The result is diabetic neuropathy, which most commonly shows up as tingling, numbness, or burning pain in the feet and hands. Over time it can progress to a loss of sensation, which is dangerous because you may not feel injuries to your feet until they become serious infections.
The Numbers That Matter
Understanding where your blood sugar falls on the spectrum helps you gauge what’s happening inside your body. Here are the standard thresholds:
Fasting blood sugar (no food for at least 8 hours):
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher on two separate tests
Two hours after a glucose tolerance test:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
Your HbA1c, a blood test that reflects your average blood sugar over the past two to three months, translates roughly like this: an HbA1c of 6% corresponds to an average blood sugar of about 126 mg/dL, 7% to about 154 mg/dL, 8% to about 183 mg/dL, and 9% to about 212 mg/dL. Each percentage point represents a meaningful jump in average glucose and, with it, a meaningful jump in risk for complications.
When High Blood Sugar Becomes an Emergency
Two acute crises can develop when blood sugar climbs dangerously high, and both are medical emergencies.
Diabetic Ketoacidosis (DKA)
DKA happens most often in type 1 diabetes, though it can occur in type 2 as well. Without enough insulin, your body can’t use glucose at all, so it starts breaking down fat at an accelerated rate. That process produces acidic byproducts called ketones. When ketones accumulate faster than your body can clear them, your blood becomes dangerously acidic. Early warning signs include worsening thirst and urination, nausea, abdominal pain, weakness, and a distinctive rapid, labored breathing pattern as your lungs try to compensate for the acid buildup. Confusion or altered consciousness can follow. Fruity-smelling breath is a classic sign that ketone levels are high.
Hyperosmolar Hyperglycemic State (HHS)
HHS is more common in type 2 diabetes and develops when blood sugar climbs above 600 mg/dL, sometimes much higher. Unlike DKA, it doesn’t produce significant ketones. Instead, the defining feature is extreme dehydration and very concentrated blood. Neurological symptoms range from lethargy and disorientation to seizures (which occur in up to 25% of cases) and coma, typically once blood concentration crosses a critical threshold. HHS develops over days to weeks and carries a high mortality rate if untreated.
What to Do When Blood Sugar Spikes
If your blood sugar reads 240 mg/dL or higher, the Mayo Clinic recommends testing your urine for ketones using an over-the-counter test kit before doing anything else. This step matters because it changes what you should do next. If ketones are present, do not exercise. Physical activity with ketones in your system can push blood sugar even higher. Instead, contact your healthcare provider, as you may need a supplemental dose of rapid-acting insulin and close monitoring.
If ketones are negative and your blood sugar is elevated but not in crisis territory, moderate physical activity like a brisk walk can help lower glucose levels. Drinking water helps counter the dehydration that high blood sugar causes. For people on insulin, a correction dose (an extra dose of short-acting insulin calculated for the situation) is the most direct way to bring levels down, but the right amount depends on your individual insulin sensitivity and should be worked out with your provider in advance.
Persistent high readings over several days, even without acute symptoms, signal that something in your management plan needs to change. That could mean adjusting medication, changing meal patterns, or identifying a hidden cause like an infection or medication interaction that’s driving glucose up.