The big picture: Glucose is a type of sugar and your principal energy source. High levels may mean you have prediabetes or diabetes and are at increased risk of developing cardiovascular disease.

Abnormal levels can indicate an issue with energy homeostasis caused by conditions that lead to hyperglycemia or hypoglycemia.

Glucose can either come from your food intake or be produced by your liver and kidneys. It is carried in our blood to all your organs and cells, where it is converted into energy.
When fasting levels of glucose fall outside of the normal range, it can indicate an issue with your body's energy homeostasis. Several conditions can affect this regulation and lead to either too much glucose (hyperglycemia) or too little glucose (hypoglycemia).

Hyperglycemia means you are progressing towards diabetes or already have it.

Those with sustained glucose levels > 126 mg/dL have diabetes. And those with glucose levels between 100 to 126 mg/dL are considered prediabetic and at high risk of progressing to diabetes.
The risk of developing cardiovascular disease is up to 4 times higher for the diabetic population.
Even if “just” prediabetic, it is already associated with a 13% increased risk of overall mortality, 16% increased risk of coronary heart disease and 14% increased risk of stroke.

The most common cause of hyperglycemia is insulin resistance.

Risk factors for the development of this condition are increased body weight, family history, poor diet, and lack of regular exercise. Another common cause is an autoimmune disease called type 1 diabetes.
In this disease, the body attacks the cells of the pancreas that produce insulin, destroying the signal for tissues to absorb glucose from the bloodstream.

Hypoglycemia may indicate that the brain is not getting enough glucose and may eventually stop functioning normally.

Hypoglycemia is a less common condition and can cause symptoms such as thumping heart, sweating, tingling, and anxiety.
If the blood glucose level drops below a certain point, the brain does not get enough glucose and stops functioning normally leading to blurred vision, difficulty concentrating, confused thinking, slurred speech, numbness, and drowsiness.
If blood glucose stays low for too long, starving the brain of glucose, it may lead to seizures, coma, and very rarely, death.