Hyperglycemia means your blood sugar is higher than normal. Specifically, it’s defined as a fasting blood glucose above 125 mg/dL or a reading above 180 mg/dL two hours after eating. Everyone’s blood sugar rises after a meal, but in hyperglycemia, it rises too high or stays elevated too long because the body can’t move glucose out of the bloodstream efficiently.
Why Blood Sugar Gets Too High
Your body relies on the hormone insulin to shuttle glucose from your blood into your cells, where it’s used for energy. Hyperglycemia happens when that system breaks down, and there are two main ways it can fail.
The first is insulin deficiency. In type 1 diabetes, the immune system destroys the cells in the pancreas that produce insulin. Without enough insulin, glucose accumulates in the blood with no way to get into cells.
The second is insulin resistance. Your cells, particularly in muscle, the liver, and fat tissue, stop responding to insulin properly. The pancreas compensates by producing more and more insulin, but eventually it can’t keep up with the demand. At that point, blood sugar starts climbing. This is the core process behind type 2 diabetes and is closely linked to excess calorie intake, visceral fat, and physical inactivity. Insulin resistance in the liver is especially problematic because the liver continues producing glucose even when blood levels are already high, essentially adding fuel to the fire.
Diabetes isn’t the only cause. Acute physical stress from surgery, serious illness, or trauma can temporarily raise blood sugar by triggering hormonal changes and making tissues less sensitive to insulin. Certain medications, including corticosteroids, can do the same.
Symptoms at Different Stages
Hyperglycemia usually doesn’t produce noticeable symptoms until blood sugar climbs above 180 to 200 mg/dL. Below that threshold, you might have elevated readings on a blood test without feeling anything at all.
Once levels push higher, the early signs include frequent urination, increased thirst, blurred vision, and feeling unusually tired or weak. These happen because your kidneys work harder to filter excess glucose, pulling more water with it and leaving you dehydrated. The blurred vision comes from fluid shifts in the lens of your eye.
If high blood sugar goes untreated, it can progress to a dangerous state called ketoacidosis, where the body starts breaking down fat for fuel and produces toxic acids called ketones. Symptoms of ketoacidosis include fruity-smelling breath, nausea and vomiting, abdominal pain, shortness of breath, confusion, and in severe cases, loss of consciousness. This is a medical emergency.
Two Emergencies to Know About
Severely uncontrolled hyperglycemia can lead to two distinct crises, and they look different from each other.
Diabetic ketoacidosis (DKA) typically comes on fast, over hours to days. Blood sugar is usually above 250 mg/dL, and the hallmark is a buildup of ketones that makes the blood dangerously acidic. It’s most common in type 1 diabetes but can occur in type 2 as well. Nausea, vomiting, abdominal pain, and rapid deep breathing are the classic warning signs.
Hyperosmolar hyperglycemic state (HHS) develops more gradually, over days to weeks. Blood sugar soars above 600 mg/dL, and severe dehydration is the dominant feature. Rather than ketone buildup, the danger comes from the blood becoming extremely concentrated. Neurological symptoms like lethargy, confusion, seizures, and coma are more prominent in HHS than in DKA. HHS occurs almost exclusively in type 2 diabetes and carries a high mortality rate.
What Chronic High Blood Sugar Does to Your Body
A single spike in blood sugar after a big meal isn’t what causes lasting harm. The real damage comes from sustained hyperglycemia over months and years. Elevated glucose gradually injures blood vessels throughout the body, and the consequences depend on which vessels are affected.
Damage to small blood vessels (microvascular disease) drives three of the most common complications of diabetes. Retinopathy affects the tiny vessels in the retina and is a leading cause of vision loss. Nephropathy damages the filtering units in the kidneys, potentially progressing to kidney failure. Neuropathy injures nerves, most often in the feet and legs, causing numbness, tingling, or pain. Poor circulation and impaired skin healing in the lower extremities can turn even minor cuts into deep, slow-healing ulcers that are prone to infection.
Damage to large blood vessels (macrovascular disease) accelerates atherosclerosis, the buildup of plaque in arteries. This raises the risk of heart attack, stroke, and peripheral artery disease. Chronic hyperglycemia also suppresses immune function directly, making infections more likely and harder to fight off.
Blood Sugar Targets for People With Diabetes
If you’ve been diagnosed with diabetes, the American Diabetes Association recommends keeping your A1C (a measure of average blood sugar over roughly three months) below 7% for most nonpregnant adults. In practical daily terms, that translates to fasting or pre-meal blood sugar between 80 and 130 mg/dL and post-meal readings below 180 mg/dL.
For people who use a continuous glucose monitor (CGM), the goal is spending more than 70% of the day in the 70 to 180 mg/dL range. Less than 5% of the day should be spent above 250 mg/dL, and less than 25% between 181 and 250 mg/dL. For older adults, these targets are somewhat more relaxed, with a time-in-range goal of more than 50%.
Bringing High Blood Sugar Down
If you check your blood sugar and find it elevated, the response depends on how high it is. Physical activity is one of the most effective ways to lower blood sugar in the short term because working muscles pull glucose from the bloodstream even without much insulin. A brisk walk after a meal can make a meaningful difference. However, if your blood sugar is above 240 mg/dL, check for ketones in your urine before exercising. Exercising with ketones present can actually make things worse.
Staying well hydrated helps your kidneys flush excess glucose. Drinking water won’t dramatically lower blood sugar on its own, but dehydration makes hyperglycemia harder for your body to manage.
For people on insulin or other glucose-lowering medications, your prescribing provider should give you a plan for adjusting doses when readings are high. Following a consistent eating pattern, managing portion sizes, and limiting refined carbohydrates are the dietary foundations of blood sugar control. None of these replace medication when it’s needed, but they influence how often and how sharply blood sugar spikes throughout the day.