High blood sugar often announces itself through a handful of telltale symptoms: excessive thirst, frequent urination, and unusual fatigue are the most common early signs. But it can also run high without any obvious warning, especially in people with type 2 diabetes or prediabetes. The only way to know for certain is to check your numbers, either with a home glucose meter or a lab test. A fasting blood sugar of 126 mg/dL or higher, or an A1C of 6.5% or above, meets the diagnostic threshold for diabetes.
The Classic Early Symptoms
The three symptoms most people notice first are intense thirst, needing to urinate far more often than usual, and feeling wiped out even after a full night’s sleep. These are all connected. When excess glucose builds up in your blood, your kidneys work harder to filter it out. Eventually they can’t keep up, and the extra sugar spills into your urine, pulling fluid from your body’s tissues along with it. That fluid loss causes dehydration, which triggers thirst. Drinking more to compensate means more trips to the bathroom. Meanwhile, because your body can’t efficiently use that glucose for energy, fatigue sets in from two directions: poor energy production and dehydration.
Blurry vision is another early sign that catches people off guard. High blood sugar pulls fluid from tissues throughout your body, including the lenses of your eyes. That changes the shape of the lenses and makes it harder to focus. This type of blurriness typically improves once blood sugar comes back down, but sustained high levels can damage the small blood vessels in the eyes over time, leading to more serious vision problems.
Unexplained weight loss can also signal high blood sugar. When glucose is leaving your body through urine instead of fueling your cells, you’re losing calories you consumed but never used. Combined with dehydration, this can cause noticeable weight loss even when you’re eating normally.
Subtle Signs You Might Miss
Not every sign of high blood sugar is dramatic. Slower, quieter changes are easy to overlook or blame on something else. Cuts and sores that take longer than usual to heal are one example. High blood sugar impairs blood flow and disrupts your body’s natural repair process, so a scrape that would normally close up in a few days may linger for weeks.
Tingling or numbness in your hands and feet signals that excess sugar is starting to affect nerve function. This doesn’t happen overnight. It’s more common when blood sugar has been running high for a prolonged period, but some people experience it before they ever receive a diabetes diagnosis.
Skin changes are another clue. Dry, itchy skin can develop because dehydration from high blood sugar pulls moisture from your cells. A more specific sign is dark, velvety patches of skin that appear in body creases like the neck, armpits, or groin. This condition, called acanthosis nigricans, is a marker of insulin resistance and often shows up in people with prediabetes or type 2 diabetes, particularly those carrying extra weight.
Red, swollen, or tender gums are less well known but worth paying attention to. High blood sugar weakens your immune defenses, making the gum tissue more vulnerable to infection.
When There Are No Symptoms at All
One of the trickiest things about high blood sugar is that it can be completely silent. People with prediabetes, type 2 diabetes, or gestational diabetes may have elevated glucose levels for months or even years without feeling anything unusual. Type 1 diabetes behaves differently: symptoms tend to come on quickly and with more intensity. But in type 2, blood sugar often creeps up gradually, and the body partially adapts to higher levels without sending obvious alarm signals.
This is why routine screening matters. If you have risk factors like a family history of diabetes, obesity, or a sedentary lifestyle, your blood sugar could be elevated right now without you knowing it. The damage from prolonged high glucose, to blood vessels, nerves, kidneys, and eyes, accumulates whether you feel symptoms or not.
How to Check at Home
A home blood glucose meter (glucometer) gives you a reading in seconds. The process is straightforward, but a few details affect accuracy. Wash your hands with soap and water and dry them completely before testing. Don’t use hand sanitizer, as residue on your skin can throw off the reading. If you use an alcohol wipe instead, let the site dry fully before pricking. Insert a fresh test strip all the way into the monitor, then use the lancet to get a generous drop of blood from the side of your fingertip. Touch the drop to the strip once. Don’t try to add more blood after the first application.
If you test somewhere other than your fingertip and the number seems off, retest using a fingertip. Fingertip readings tend to be the most reliable, especially when blood sugar is changing rapidly, like right after a meal or exercise.
What the Numbers Mean
For a fasting test (no food for at least eight hours), normal blood sugar falls below 100 mg/dL. Between 100 and 125 mg/dL is considered prediabetes. At 126 mg/dL or above on two separate occasions, the result meets the threshold for a diabetes diagnosis.
After eating, blood sugar naturally rises. A reading under 180 mg/dL two hours after the start of a meal is the general target for people managing diabetes. In people without diabetes, post-meal glucose typically stays well below that mark, usually under 140 mg/dL.
A1C is a lab test that reflects your average blood sugar over the previous two to three months. An A1C of 6.5% or higher indicates diabetes. This test captures the bigger picture, so even if a single fasting reading looks acceptable, an elevated A1C can reveal that blood sugar has been running high overall.
Emergency Warning Signs
When blood sugar climbs dangerously high, especially in combination with a shortage of insulin, the body starts breaking down fat for fuel and produces acidic byproducts called ketones. This condition, diabetic ketoacidosis, can become life-threatening quickly. The warning signs are distinct and hard to ignore:
- Fruity-smelling breath, caused by ketones being expelled through the lungs
- Nausea, vomiting, or stomach pain that won’t let up
- Fast, deep breathing as the body tries to correct rising acid levels in the blood
- Dry mouth and flushed face
- Severe fatigue or confusion
A blood sugar reading that stays at or above 300 mg/dL, breath that smells fruity, or vomiting that prevents you from keeping food or fluids down are all reasons to get to an emergency room or call 911. This is especially relevant for people with type 1 diabetes, but it can occur in type 2 as well during illness or severe insulin deficiency.