High blood sugar, called hyperglycemia, means there is more glucose circulating in your bloodstream than your body can use. The clinical thresholds are a fasting blood sugar above 125 mg/dL or a reading above 180 mg/dL two hours after eating. A single high reading doesn’t necessarily mean you have diabetes, but consistently elevated numbers signal that something in your body’s glucose regulation isn’t working properly.
Why Blood Sugar Rises
Your body runs on glucose, a sugar that comes from the food you eat. Normally, your pancreas releases insulin, a hormone that acts like a key to unlock your cells so glucose can enter and be used for energy. When that system breaks down, glucose stays trapped in the bloodstream and levels climb.
There are two main ways this happens. In type 1 diabetes, the pancreas produces little or no insulin, so there’s simply no key available. In type 2 diabetes, your cells stop responding properly to insulin, a condition called insulin resistance. Your pancreas tries to compensate by producing more, but eventually can’t keep up. The result is the same: glucose builds up in the blood instead of fueling your cells.
Diabetes isn’t the only cause. Physical stress from illness, injury, or surgery can spike blood sugar even in people without diabetes. During a stress response, your adrenal glands can produce up to ten times their normal output of cortisol and other stress hormones. These hormones trigger your liver to dump stored glucose into the bloodstream and simultaneously make your cells more resistant to insulin. Certain medications, particularly steroids, can do the same thing. Infections, lack of sleep, and dehydration are other common triggers.
How High Blood Sugar Feels
Mild hyperglycemia often produces no noticeable symptoms, which is part of what makes it dangerous. Many people walk around with elevated glucose for months or years before getting a diagnosis. When symptoms do appear, they tend to follow a predictable pattern.
Excessive thirst and frequent urination are the hallmark signs. When blood sugar is high, your kidneys work overtime to filter out the extra glucose, pulling water along with it. That makes you urinate more, which dehydrates you, which makes you intensely thirsty. You might also notice blurred vision, unusual fatigue, and increased hunger, even if you’re eating regularly. The hunger happens because your cells aren’t getting the glucose they need for energy, so your body sends signals to eat more, even though there’s plenty of sugar in your blood already.
The Numbers That Matter
Blood sugar is measured in a few different ways, and each has its own thresholds for normal, prediabetic, and diabetic ranges.
- Fasting blood sugar: Normal is below 100 mg/dL. Between 100 and 125 mg/dL falls in the prediabetic range. At 126 mg/dL or higher on two separate tests, it meets the criteria for diabetes.
- A1C: This blood test reflects your average blood sugar over the past two to three months. An A1C between 5.7% and 6.4% indicates prediabetes. At 6.5% or higher, it indicates diabetes.
- Post-meal reading: Blood sugar above 180 mg/dL two hours after eating is considered hyperglycemic.
If you use a continuous glucose monitor, the goal for most people with type 1 or type 2 diabetes is to spend at least 70% of the day in the target range of 70 to 180 mg/dL. That works out to roughly 17 hours a day. For older adults or those at higher risk of complications, the target is more relaxed at 50% or about 12 hours. Even a 5% improvement in time spent in range is associated with meaningful health benefits.
Why Morning Readings Can Be Surprisingly High
If you’ve ever woken up to a high fasting number despite not eating anything overnight, you’ve likely experienced the dawn phenomenon. In the early morning hours, your body releases a surge of growth hormone and cortisol that naturally raises blood sugar. Your liver also ramps up glucose production through a combination of breaking down stored sugar and manufacturing new glucose.
In people without diabetes, the pancreas simply releases more insulin to counteract this. But if you have diabetes or insulin resistance, there isn’t enough insulin to offset the surge. If you take insulin, its effects may also be wearing off by early morning, leaving the liver’s glucose output unopposed.
What Happens When It Stays High
A temporary spike from a large meal or a stressful day isn’t likely to cause lasting harm. The real damage comes from chronically elevated blood sugar over months and years. Excess glucose in the bloodstream generates harmful molecules called advanced glycation end products and reactive oxygen species. These damage the walls of blood vessels, both large and small, throughout the body.
The small blood vessel damage tends to hit three organs hardest. In the kidneys, it leads to diabetic kidney disease. About 25% of people with type 2 diabetes develop early signs of kidney damage within 10 years of diagnosis. In the eyes, it causes diabetic retinopathy, which accounts for roughly 10,000 new cases of blindness each year in the United States alone. Nerve damage, particularly in the feet and hands, is the third common target, causing numbness, tingling, and pain.
Large blood vessel damage accelerates atherosclerosis, the buildup of plaque in arteries. People with diabetes and existing small vessel complications face a particularly elevated risk of heart attack and stroke. This is why managing blood sugar isn’t just about avoiding symptoms today. It’s about protecting your cardiovascular system, kidneys, and vision over the long term.
When High Blood Sugar Becomes an Emergency
Most high readings can be managed at home, but certain situations require immediate medical attention. If your blood sugar reaches 250 mg/dL or above, you should check your urine for ketones every four to six hours. Ketones are acids your body produces when it starts burning fat for fuel instead of glucose, and elevated levels indicate diabetic ketoacidosis (DKA), a life-threatening condition.
Call 911 or go to the emergency room if your blood sugar stays at 300 mg/dL or above, your breath smells fruity (a sign of ketones), you’re vomiting and can’t keep fluids down, or you’re having difficulty breathing. DKA develops most often in people with type 1 diabetes but can occur in type 2 as well, particularly during illness.
Practical Steps for a High Reading
If you check your blood sugar and it’s elevated, the two most immediately helpful things you can do are drink water and move your body. Hydration helps your kidneys flush excess glucose through urine. Light to moderate physical activity, like a walk, helps your muscles pull glucose out of the bloodstream for energy.
The timing and intensity of exercise matter. Starting a light walk or other aerobic activity about 30 minutes after a meal can significantly blunt the post-meal glucose surge with minimal risk of dropping your blood sugar too low. A 20 to 30 minute session of moderate activity, or 60 minutes of light activity like casual walking, is typically enough to see a noticeable difference. Adding resistance exercises like bodyweight squats or light weights two to three times a week provides additional benefit over time.
One important caveat: if your blood sugar is above 250 mg/dL and you have ketones in your urine, exercise can actually make things worse. In that scenario, focus on hydration and contact your healthcare team. If you take insulin, you may have a correction dose protocol to follow for high readings.
Prediabetes: The Warning Window
If your numbers fall in the prediabetic range (fasting glucose 100 to 125 mg/dL or A1C 5.7% to 6.4%), your body is already struggling with insulin resistance but hasn’t crossed the threshold into full diabetes. This is actually the most important window for action, because prediabetes is often reversible with lifestyle changes. Modest weight loss of 5% to 7% of body weight, regular physical activity, and dietary adjustments that reduce refined carbohydrates can bring blood sugar back into the normal range and significantly reduce the risk of progressing to type 2 diabetes.