High blood sugar often announces itself through a handful of telltale signs: unusual thirst, frequent urination, and blurred vision are among the earliest. But here’s the catch: blood sugar can run high for months or even years without producing any obvious symptoms at all. About 1 in 3 adults has prediabetes, and most of them don’t know it. Recognizing the warning signs matters, but so does understanding when and how to test, because your body won’t always alert you on its own.
Early Symptoms of High Blood Sugar
When blood sugar climbs above normal levels, the first symptoms tend to revolve around fluid. Your kidneys work to flush excess glucose out through urine, pulling extra water along with it in a process called osmotic diuresis. That’s why frequent urination is usually the earliest sign. Your body then tries to compensate for the fluid loss by ramping up thirst, sometimes dramatically. You may find yourself drinking far more water than usual and still feeling parched.
Other early symptoms include:
- Increased hunger, even shortly after eating
- Headaches
- Blurred vision
If blood sugar stays elevated over weeks or months, a second wave of symptoms can develop. These include persistent fatigue, unexplained weight loss, slow-healing cuts or sores, recurring skin infections, and vaginal yeast infections. These longer-term signs often appear gradually enough that people attribute them to stress, aging, or poor sleep before considering blood sugar as the cause.
When High Blood Sugar Has No Symptoms
One of the trickiest things about blood sugar is that moderately elevated levels frequently produce no symptoms whatsoever. This is especially true in prediabetes, where blood sugar is above normal but not yet in the diabetic range. Most people with prediabetes feel completely fine. The damage, though, can still accumulate quietly in blood vessels, nerves, and organs. This is why routine blood work matters even if you feel healthy, particularly if you have risk factors like a family history of diabetes, excess weight, or a sedentary lifestyle.
What the Numbers Actually Mean
If you’re checking blood sugar at home with a glucose meter, there are two key windows to pay attention to. Before meals (fasting), a normal reading falls between 80 and 130 mg/dL. One to two hours after eating, blood sugar should stay below 180 mg/dL. Readings consistently above these thresholds suggest your blood sugar is running too high.
For a longer-term picture, a test called A1C measures your average blood sugar over roughly three months. The thresholds break down like this:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
A single high reading on a home meter doesn’t necessarily mean you have diabetes. Stress, illness, certain medications, and even a large meal can temporarily spike blood sugar. Patterns over time are what matter. If you’re seeing repeated fasting readings above 130 mg/dL or post-meal readings above 180, that’s worth investigating further with your doctor through an A1C test or a fasting plasma glucose test.
How to Check at Home
A basic glucose meter (glucometer) is the most accessible way to spot-check your blood sugar. The process takes about a minute. Wash your hands with warm water and soap first, since residue from food or lotion can throw off the reading. Dry your hands thoroughly. If your fingers are cold, warm them up or shake out your hand to improve blood flow to your fingertips.
Insert a fresh test strip into the meter, then use a lancet to prick the side of your fingertip. Gently squeeze from the base of the finger to produce a small drop of blood, touch it to the test strip, and wait a few seconds for the number to appear. Record your result along with the time, what you ate, and anything else that might have affected the reading, like exercise or stress. Never share lancets with anyone else, even family members.
The most useful times to test are first thing in the morning before eating (your fasting level) and one to two hours after a meal (to see how your body handled the food). Testing at both windows gives you a much clearer picture than either one alone.
Emergency Signs That Need Immediate Attention
Very high blood sugar, typically above 250 mg/dL, can trigger a dangerous condition called diabetic ketoacidosis (DKA). This happens when the body can’t use glucose for energy and starts breaking down fat at an accelerated rate, producing acids called ketones that build up in the blood. DKA is most common in people with type 1 diabetes, but it can occur in type 2 as well.
Early signs of DKA overlap with regular high blood sugar symptoms: extreme thirst, frequent urination, intense hunger, and signs of dehydration like dry mouth and flushed skin. As it progresses, more alarming symptoms appear:
- Nausea and vomiting
- Abdominal pain
- Rapid, deep breathing
- Fruity-smelling breath
- Confusion or disorientation
- Extreme fatigue or decreased alertness
Fruity-smelling breath is one of the most distinctive warning signs. It’s caused by acetone, a byproduct of the ketone buildup. If you or someone around you notices this smell along with any of the symptoms above, that situation requires emergency care. DKA can lead to loss of consciousness and is life-threatening without treatment.
Who Should Be Checking
If you already have a diabetes diagnosis, regular home monitoring is part of managing your condition. How often you check depends on your type of diabetes, your medications, and your treatment plan. But home testing isn’t only for people with a diagnosis. If you’re experiencing symptoms like unexplained thirst, frequent urination, or fatigue, a glucose meter can help you gather useful data before a doctor’s visit.
Even without symptoms, periodic screening through standard blood work is important for adults over 35, people with a BMI in the overweight or obese range, those with a family history of type 2 diabetes, and anyone who has had gestational diabetes. Since prediabetes is so common and so silent, screening is often the only way to catch blood sugar problems before they progress to full diabetes, when the damage is harder to reverse.