Fasting hyperglycemia is when your blood sugar levels stay high even when you haven’t eaten for a long time. Doctors may check your fasting blood sugar levels to help diagnose various conditions, such as diabetes.

When you eat, your body breaks down your food into an energy source called glucose, or blood sugar. Most people will experience a drop in blood sugar when they don’t eat or drink beverages with calories for several hours.

For some, their blood sugar still stays high for a long time. This is called fasting hyperglycemia.

If your blood sugar levels get higher when you haven’t eaten for 2 hours or more, this may be a signal to talk with your healthcare team about prediabetes, diabetes, or possibly another health issue that’s raising your glucose levels.

Here’s what to know about fasting hyperglycemia, why you may experience it, and what you can do about it.

Usually, when you fast, your body uses the sugar in your blood to fuel your cells and organs. That causes it to dip.

But sometimes, your blood sugar goes up even when you fast.

It could be because your body isn’t making enough insulin, a hormone that allows your cells to absorb the sugar. Or it could be because your body has become insulin resistant. It could also be because your body is making too much sugar.

This could be because a condition such as diabetes or prediabetes is affecting your body’s ability to use sugar. Other conditions can also cause it.

Other causes of hyperglycemia include:

  • problems with an organ called the pancreas, which makes insulin
  • endocrine disorders such as Cushing syndrome and acromegaly
  • some medications, such as estrogens and corticosteroids
  • gestational diabetes (having diabetes when you’re pregnant)
  • a reaction after surgery or if you’re critically ill

Symptoms of hyperglycemia can include:

  • dizziness
  • fatigue
  • thirst
  • blurry vision
  • headache
  • the need to urinate more often

Some people have no symptoms other than the high blood sugar level shown on a test.

Fasting hyperglycemia doesn’t always mean you have diabetes.

You can have it for other reasons, such as a hormone imbalance, a problem with your pancreas, illness, surgery, or another reason. It’s important to work with a doctor to figure out the cause.

A doctor will use the following glucose levels to help figure out whether higher blood sugars are a symptom of developing diabetes:

Fasting doesn’t usually cause high blood sugar, but it’s possible.

If you already have diabetes, you may experience an early morning increase in blood sugar, known as the dawn phenomenon. This is when a surge of nighttime hormones causes a blood sugar spike in people with diabetes, but this doesn’t affect those without diabetes.

If doctors find that you have fasting hyperglycemia, they’ll try to figure out why. The cause determines your treatment.

For instance, if you have type 1 diabetes, they’ll recommend a nutrition plan and prescribe insulin to help your body use blood sugar properly. They’ll give you ways to check your blood sugar.

If you have type 2 diabetes, your doctor may suggest lifestyle and diet changes and may give you different types of medication. They’ll also give you ways to check your blood sugar levels to figure out if treatment is working.

They may also give you more tests to figure out if high blood sugar has damaged other parts of your body. Then they’ll help you decide whether you need further treatment.

Other conditions may require different types of treatment. Your doctor can provide more information about what treatments they recommend after diagnosis.

Most people don’t have high blood sugar levels after several hours of fasting. When you have high blood sugar levels even though you haven’t eaten or consumed calorie-containing beverages, it’s called fasting hyperglycemia.

A doctor may recommend a test to determine whether you have fasting hyperglycemia. It can happen due to conditions such as prediabetes, diabetes, the use of some medications, and hormone imbalances.

High blood sugar levels can damage cells, tissues, and organs over time. It’s important to know if you have hyperglycemia so you can work with a doctor to determine the causes and suitable treatments. Speak with a doctor if you experience dizziness, thirst, blurry vision, fatigue, or other symptoms of hyperglycemia.