High blood sugar often develops gradually, and many people have it for months or years without realizing it. The earliest clues are usually increased thirst, frequent urination, and unexplained fatigue. But symptoms alone aren’t reliable, especially in the early stages. A fasting blood sugar of 100 mg/dL or higher already puts you in prediabetes territory, and many people at that level feel completely normal.
The Most Common Early Signs
The CDC lists these as the hallmark symptoms of elevated blood sugar:
- Frequent urination, especially waking up multiple times at night to use the bathroom
- Increased thirst that doesn’t go away no matter how much water you drink
- Unusual hunger, even shortly after eating
- Unexplained weight loss
- Fatigue that feels disproportionate to your activity level
- Blurry vision
- Irritability or mood changes
- Frequent UTIs or yeast infections
The urination and thirst tend to appear together for a straightforward reason. When blood sugar climbs too high, your kidneys can’t reabsorb all the excess glucose, so they pull extra water into the urine to flush it out. That leaves you dehydrated, which triggers intense thirst. You drink more, you urinate more, and the cycle continues. This is why the bathroom-and-water-bottle pattern is often the first thing people notice.
Blurry vision happens because high glucose can change the shape of the lens in your eye and cause deposits to build up, making things look cloudy or out of focus. This can come and go as blood sugar fluctuates, which is why some people assume they just need new glasses rather than suspecting a blood sugar problem.
Why You Might Have No Symptoms at All
Here’s the tricky part: prediabetes and early type 2 diabetes frequently produce zero noticeable symptoms. Blood sugar can sit in the 100 to 150 mg/dL range for years, quietly damaging blood vessels and nerves, without triggering the obvious signs listed above. The dramatic symptoms, like constant thirst and rapid weight loss, typically show up once levels are significantly and consistently elevated. That’s why testing matters even if you feel fine, particularly if you have risk factors like a family history of diabetes, excess weight around the midsection, or a sedentary lifestyle.
Signs That Blood Sugar Is Dangerously High
When blood sugar stays very high (generally above 240 mg/dL) and the body can’t use glucose for energy, it starts breaking down fat instead, producing chemicals called ketones. This can spiral into a medical emergency called diabetic ketoacidosis. The warning signs include:
- Nausea and vomiting
- Belly pain
- Shortness of breath
- Fruity-scented breath
- Confusion or difficulty thinking clearly
- Extreme weakness
If you or someone around you has several of these symptoms at the same time, that requires immediate emergency care. Diabetic ketoacidosis is most common in type 1 diabetes but can happen in type 2 as well, especially during illness or infection.
How Blood Sugar Is Tested
The only way to confirm high blood sugar is with a blood test. There are several types, each measuring something slightly different.
Fasting Blood Sugar
You fast overnight, then have blood drawn in the morning. A normal result is below 100 mg/dL. Between 100 and 125 mg/dL is prediabetes. At 126 mg/dL or higher, it’s diabetes. This is one of the most common screening tests because it’s simple and inexpensive.
A1C Test
This measures your average blood sugar over the past two to three months, expressed as a percentage. Below 5.7% is normal. Between 5.7% and 6.4% is prediabetes. At 6.5% or higher, it’s diabetes. The advantage of A1C is that it doesn’t require fasting and captures the bigger picture rather than a single snapshot.
Oral Glucose Tolerance Test
You drink a sugary solution, then your blood sugar is measured two hours later. Below 140 mg/dL is normal. Between 140 and 199 mg/dL is prediabetes. At 200 mg/dL or higher, it’s diabetes. This test is especially useful for catching blood sugar problems that only show up after eating.
Random Blood Sugar Test
A blood draw taken at any time, regardless of when you last ate. A result of 200 mg/dL or higher, combined with symptoms, suggests diabetes.
Checking Blood Sugar at Home
If you already have a glucose meter (or want to buy one over the counter), you can get a reading in under a minute. Wash your hands with warm, soapy water and dry them well. Cold hands can restrict blood flow to your fingertips and make it harder to get a good sample, so warming them up first helps. Use the lancet to prick the side of your fingertip, gently squeeze a drop of blood onto the test strip, and insert the strip into the meter. The reading appears in a few seconds.
A single high reading doesn’t mean you have diabetes. Blood sugar naturally rises after meals, during stress, and when you’re fighting off an illness. What matters is the pattern over time. If you’re consistently seeing fasting numbers above 100 mg/dL, or numbers above 200 mg/dL at any point, those results warrant a proper lab test to confirm what’s going on.
Who Should Get Tested
The American Diabetes Association recommends screening for anyone 35 or older, repeated every three years if results are normal. Testing should happen earlier or more frequently if you have additional risk factors: a BMI of 25 or higher, a parent or sibling with diabetes, a history of gestational diabetes, polycystic ovary syndrome, or if you belong to a racial or ethnic group with higher diabetes rates (including Black, Hispanic, Native American, Asian American, and Pacific Islander populations).
If you’re experiencing the classic symptoms, like unquenchable thirst paired with constant trips to the bathroom, don’t wait for your next scheduled checkup. A fasting glucose or A1C test can give you a clear answer quickly, and catching high blood sugar early makes a significant difference in how effectively it can be managed.