High blood sugar often doesn’t cause noticeable symptoms until levels climb above 180 to 200 mg/dL, which means you can walk around with elevated glucose for weeks or months without realizing it. The signs that do eventually appear tend to build gradually, making them easy to dismiss as stress, poor sleep, or aging. Knowing what to look for, and what your numbers actually mean, can help you catch the problem early.
The Early Warning Signs
The classic trio of high blood sugar symptoms is increased thirst, frequent urination, and fatigue. When glucose builds up in your bloodstream, your kidneys work harder to filter it out, pulling extra water along with it. That’s why you find yourself urinating more often, especially at night, and feeling thirsty no matter how much you drink. The cycle leaves you dehydrated and drained of energy even after a full night’s rest.
Other common early signs include blurred vision, unexplained weight loss, and slow-healing cuts or sores. High blood sugar changes the shape of the lenses in your eyes and can cause deposits to build up inside them, making your vision cloudy or unfocused. Wounds heal more slowly because excess glucose impairs circulation and the body’s ability to repair tissue. Tingling or numbness in your hands and feet, frequent infections (especially yeast infections or urinary tract infections), and dry, itchy skin round out the list of symptoms that tend to appear before anything feels truly urgent.
When High Blood Sugar Is Silent
One of the trickiest things about elevated glucose is that it can cause zero symptoms for a long time. People who have had type 2 diabetes for years may not show any signs despite persistently high levels. This is especially common in prediabetes, where blood sugar is above normal but hasn’t crossed the diabetes threshold. Without routine testing, there’s no way to know.
Repeated episodes of high blood sugar also stress the brain over time, and the effects aren’t obvious right away. Chronically elevated glucose damages blood vessels that carry oxygen to brain cells, which can gradually affect memory, concentration, and thinking speed. People often don’t realize their brain is being affected until the damage has accumulated.
Physical Changes You Can See
Chronic insulin resistance, the metabolic problem behind most cases of high blood sugar, sometimes leaves visible clues on your skin. Acanthosis nigricans is a condition that causes patches of dark, thick, velvety skin in body folds and creases, most commonly the back of the neck, armpits, and groin. It develops slowly and may be itchy or develop small skin tags. People who have acanthosis nigricans are much more likely to develop type 2 diabetes, so these patches are worth mentioning to a doctor even if you feel fine otherwise.
What the Numbers Mean
The only way to confirm high blood sugar is to measure it. Here are the standard thresholds used for diagnosis:
- Fasting blood glucose: Normal is below 100 mg/dL. Between 100 and 125 mg/dL is prediabetes. At 126 mg/dL or higher, it’s diabetes.
- Two hours after a glucose drink (oral glucose tolerance test): Normal is below 140 mg/dL. Between 140 and 199 mg/dL is prediabetes. At 200 mg/dL or higher, it’s diabetes.
- Random blood glucose: A reading of 200 mg/dL or higher at any time, combined with symptoms, is enough for a diabetes diagnosis.
What A1C Tells You
An A1C test measures your average blood sugar over roughly the past two to three months. It’s reported as a percentage: below 5.7% is normal, 5.7% to 6.4% is prediabetes, and 6.5% or higher is diabetes. Each percentage point translates to a specific average glucose level. An A1C of 6% corresponds to an average of about 126 mg/dL. At 7%, that average climbs to 154 mg/dL. At 9%, you’re looking at an average around 212 mg/dL. This test is useful because it captures the bigger picture rather than a single snapshot.
How to Check at Home
Most people who monitor blood sugar at home use a fingerstick glucose meter. You prick your fingertip, place a drop of blood on a test strip, and get a reading in seconds. It’s accurate and inexpensive, though it only shows your level at that exact moment.
Continuous glucose monitors (CGMs) offer a more detailed picture. A small sensor sits just under the skin, typically on the back of your upper arm or abdomen, and measures glucose in the fluid between your cells. Once placed, the sensor can be worn for 10 to 14 days, transmitting readings every 1 to 15 minutes to your phone or a receiver. CGMs are especially helpful for spotting patterns, like overnight spikes or post-meal surges, that a single fingerstick would miss. They were once reserved for people on insulin, but access has expanded significantly and some are now available over the counter.
What Pushes Blood Sugar Up
If you already have diabetes or prediabetes, many things beyond food can raise your levels. Illness and infection are common triggers because the body releases stress hormones that push glucose higher. Emotional stress from work, relationships, or major life changes does the same thing. Being inactive, even for a few days, reduces your muscles’ ability to absorb glucose from the bloodstream. Certain medications, particularly steroids and immunosuppressants, can raise blood sugar substantially. Surgery, injury, and even poor sleep contribute.
For people on insulin or other diabetes medications, skipping doses, using expired insulin, or injecting incorrectly can all lead to spikes. These causes are worth knowing because they explain why your numbers might suddenly jump even when your diet hasn’t changed.
Emergency Signs That Need Immediate Attention
Very high blood sugar can lead to a dangerous condition called diabetic ketoacidosis, which develops when the body starts breaking down fat for fuel and produces acids called ketones at a toxic rate. The symptoms escalate quickly: fast, deep breathing; fruity-smelling breath; nausea and vomiting; stomach pain; flushed face; extreme fatigue; and muscle stiffness. Dry skin and mouth, along with a persistent headache, are also common.
The CDC recommends going to the emergency room or calling 911 if your blood sugar stays at 300 mg/dL or above, your breath smells fruity, you’re vomiting and can’t keep food or fluids down, or you’re having difficulty breathing. Ketoacidosis is most common in type 1 diabetes but can occur in type 2 as well, particularly during illness or when medication is missed. It progresses fast and is life-threatening without treatment.