Fasting Glucose: Normal Range, What High & Low Levels Mean

A fasting glucose test measures the concentration of glucose — the sugar your body uses for energy — in your blood after you've gone at least eight hours without eating or drinking anything other than water. Your doctor uses this number to get a clear, food-free baseline of how well your body is regulating blood sugar. Because eating temporarily raises glucose levels, fasting before the test removes that variable and makes the result much more meaningful. It's a standard tool for screening for diabetes and prediabetes, and for monitoring people who already have a diagnosis. Talk with your healthcare provider about what your specific result means in the context of your overall health.

According to the American Diabetes Association, a normal fasting glucose level for adults falls between 70 and 99 mg/dL. A result in this range suggests your body is processing and regulating blood sugar efficiently. When the number rises to between 100 and 125 mg/dL, it falls into the prediabetes range. A reading of 126 mg/dL or higher on two separate tests meets the diagnostic threshold for diabetes. Below 70 mg/dL may indicate low blood sugar, which doctors call hypoglycemia. These ranges apply to adults; your healthcare provider can tell you how your specific number fits your individual health picture.

Several things can push your fasting glucose above the normal range. The most common cause is type 2 diabetes, but type 1 diabetes, prediabetes, and insulin resistance can all raise the number. Other possible causes include gestational diabetes during pregnancy, polycystic ovary syndrome (PCOS), Cushing's syndrome (a condition involving excess cortisol), pancreatitis or other pancreatic disease that reduces insulin production, and certain medications such as corticosteroids, antipsychotics, or thiazide diuretics. Even significant physical stress or acute illness can temporarily elevate glucose through counterregulatory hormones. A single elevated reading doesn't automatically mean a diagnosis — your healthcare provider can interpret your result alongside your full medical history.

Fasting glucose below 70 mg/dL is called hypoglycemia, and it has a range of potential causes. Excess insulin — whether from medication or, rarely, a tumor called an insulinoma — is one possibility. Skipping or delaying meals, intense prolonged exercise without enough carbohydrates, and alcohol consumption (especially without food) can all lower blood sugar. Certain oral diabetes medications such as sulphonylureas, liver disease, adrenal insufficiency (Addison's disease), and a condition called reactive hypoglycemia, where blood sugar drops two to four hours after eating, are other known causes. A low fasting reading warrants attention, so bring your result to your healthcare provider so they can determine what's driving it.

High blood sugar (hyperglycemia) can cause increased thirst, frequent urination, blurred vision, fatigue, weakness, and slow-healing wounds or infections. With long-standing high glucose, you might also notice numbness or tingling in your hands and feet, and people with type 1 diabetes may experience unintentional weight loss. Low blood sugar (hypoglycemia) tends to feel more immediate — shakiness, sweating, a rapid heartbeat, hunger, dizziness, irritability, or difficulty concentrating. Pale skin is another common sign. In severe cases, low blood sugar can lead to seizures or loss of consciousness. Because symptoms can overlap with other conditions, your healthcare provider is the right person to evaluate what you're experiencing alongside your test result.

A result between 100 and 125 mg/dL places you in the prediabetes range, which is meaningful information your doctor can help you act on. When you meet with your provider, you might ask whether any repeat testing is recommended to confirm the result, whether additional tests like HbA1c would give a fuller picture of your blood sugar trends, and what factors specific to your lifestyle or medications might be contributing. You could also ask how frequently your glucose should be monitored going forward. Coming prepared with your questions helps you get the most from the conversation. Your healthcare provider is best placed to guide next steps based on your complete medical history and goals.