Key Takeaways
Hyperglycemia (high blood sugar) is an early warning sign of diabetes. It causes symptoms like excessive thirst and hunger, increased urination, and fatigue.
Symptoms develop gradually and may go unnoticed until more serious complications develop. Left untreated, it can damage blood vessels and nerves and may eventually harm the eyes, heart, kidneys, and feet.
If you’re having symptoms of high blood sugar, it’s best to see your provider, whether you have already been diagnosed with diabetes or not.
Hyperglycemia (high blood sugar) can develop gradually, with mild early symptoms like fatigue and thirst that may go unnoticed for years. Over time, high blood sugar can damage blood vessels and lead to serious complications affecting the eyes, heart, kidneys, and feet.1

1. Excessive Thirst
   Your kidneys help regulate blood sugar by filtering and reabsorbing glucose, so healthy urine contains little to no glucose. When blood sugar is too high, the kidneys work harder to remove the excess, pulling fluids from body tissues to dilute and flush out the sugar.2

This leads to dehydration and increased thirst. Persistent dry mouth or feeling extremely thirsty, despite drinking a lot, can be a sign of hyperglycemia.23

2. Increased Urination
   More frequent trips to the bathroom, especially at night, can be a sign of high blood sugar. This happens because your kidneys are drawing extra water out of your tissues to dilute the excess sugar in your blood and eliminate it through urine.4
3. Increased Hunger
   When there’s extra sugar in your bloodstream, it means that your body cannot use it for fuel. Your cells end up starved of energy, which causes you to feel hungrier than usual. However, what you eat matters because the more carbohydrates you consume, the higher your blood sugar will be.4
4. Blurry Vision
   High sugar levels force the body to pull fluid from your tissues, including the lenses of your eyes. This can affect your ability to focus on what you’re looking at and may lead to blurry vision.4
5. Fatigue
   Typically, your cells absorb blood sugar for energy. When sugar stays in the blood instead of being taken into your cells, your cells don’t have enough. This can make you feel sluggish or fatigued, especially after you’ve eaten a high-carb meal.4
6. Infections
   If your immune system is not working well as a result of high blood sugar and stress on the body, you might be more likely to get infections. It’s also thought that some kinds of bacteria actually live best in high-sugar environments, so they might thrive in your body when your levels are high.5

Since yeast feeds on sugar, having elevated blood sugar levels can lead to an overgrowth of yeast. For people with a vagina, frequent yeast infections are common if they have high blood sugar levels.6

7. Skin Conditions
   Dry, itchy skin, and wounds or cuts that are slow to heal can be a sign of hyperglycemia. Another sign that points to insulin resistance is acanthosis nigricans, a condition in which thick, velvety patches form in the folds or creases of areas like the neck.

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8. An illustration of a person running on a track

8. Worsened Mood
   Researchers also believe that your blood sugar levels may impact your mood. While more research is needed, there is some evidence that higher blood sugar levels after meals in people with type 2 diabetes might be linked to certain negative mood symptoms, like irritability.8

Other studies have suggested that having higher blood sugar levels might be linked to feelings of anger and sadness among people with diabetes.9

9. Stomach Pain
   Chronic hyperglycemia can lead to nerve damage in the stomach (gastroparesis or delayed gastric emptying). This condition can cause nausea and very slow digestion. Stomach pain can also be a sign of diabetic ketoacidosis, a medical emergency that needs to be treated right away.10
10. Unintended Weight Loss
    Unintentional weight loss is an important sign of elevated blood sugar, particularly in kids who are drinking and urinating often. Many children diagnosed with type 1 diabetes lose weight before they’re diagnosed because their bodies cannot use the sugar in the bloodstream for fuel.11
11. Numbness
    Nerve damage in the extremities (peripheral neuropathy), can happen over time in people with high blood sugar. It causes symptoms like numbness, tingling, or pain in the hands, feet, or legs.12

Complications
Chronic hyperglycemia can lead to a host of complications known as micro (small) and macro (large) vascular issues. They include damage to the:13

Eye (retinopathy)
Kidney (nephropathy)
Peripheral and autonomic neuropathy (nerve loss in the feet and other areas of the body, such as the intestine)
Additionally, chronically elevated blood sugars can cause or worsen:

Heart disease and peripheral arterial disease: In peripheral arterial disease, narrowed arteries reduce blood flow to the arms and legs.
Hyperglycemic hyperosmolar nonketotic coma (HHNKC): Most common in type 2 diabetes, this potentially life-threatening condition involves blood sugar over 600 mg/dL, with symptoms like extreme thirst, confusion, high fever, weakness, or one-sided paralysis. Left untreated, it can lead to coma or death.142
Diabetic ketoacidosis (DKA): Often seen in individuals with type 1 diabetes, DKA occurs when the body lacks insulin, resulting in acidic blood and elevated glucose levels. It often begins with intense thirst and frequent urination, but can develop into more severe symptoms. DKA requires immediate medical attention.1510
Erectile dysfunction: High blood sugar can damage blood vessels, increasing the risk of erectile dysfunction.16
Pregnancy complications: Poorly controlled diabetes raises certain pregnancy risks, including early pregnancy loss, preeclampsia (uncontrolled blood pressure in pregnant people), and low blood sugar in newborns.17 The ADA advises people with diabetes to aim for an A1C below 6.5% before getting pregnant.18
When to See a Healthcare Provider
It’s important to contact a healthcare provider in the following situations:

Frequent high blood sugar readings: If your levels remain elevated for several consecutive days, contact your healthcare provider. It may be a sign that your treatment needs adjusting.
New symptoms with no diabetes diagnosis: If you’re experiencing symptoms of high blood sugar, especially with risk factors like being overweight or having a family history of diabetes, schedule a screening.
Excessive drinking, eating, and urinating or weight changes in children: See a provider if your child is suddenly drinking, eating, or urinating more than usual, or if there’s a rapid weight change. If their blood sugar is over 240 mg/dL and ketone levels are high, go to the ER.10