Symptoms of High Blood Sugar

High blood sugar, or hyperglycemia, happens when there's too much glucose in the bloodstream. It's most often caused by diabetes, but it can also happen without diabetes. Symptoms range from nothing at all to a medical emergency, depending on how high the glucose is, how fast it rose, and the type of diabetes involved.

Early Symptoms

In the early stages, high blood sugar is often mild enough that nothing feels wrong. When early symptoms do appear, they tend to be subtle:

• Feeling a little more thirsty than usual
• Needing to urinate slightly more often, including at night
• Mild fatigue
• Headache

These early signs are easy to dismiss or attribute to something else, which is part of why high blood sugar can go unnoticed, especially when it rises gradually.

Common Symptoms

As blood sugar stays high or climbs further, the classic symptoms become more noticeable:

• Increased thirst
• Frequent urination
• Fatigue
• Blurred vision
• Headache
• Slow-healing cuts or wounds
• Unexplained weight loss

These symptoms often develop gradually, especially in type 2 diabetes, which is part of why high blood sugar can go unrecognized for a long time. The American Diabetes Association lists high blood glucose, high glucose in the urine, frequent urination and increased thirst among the key signs to watch for.

Symptoms in Type 1 vs. Type 2 Diabetes

High blood sugar can look different depending on the type of diabetes behind it.

• Type 1 diabetes: according to the CDC, people in the early stages of type 1 often have no symptoms at all. Once the pancreas stops producing enough insulin, symptoms can appear suddenly, over just a few weeks, and can become severe quickly — including nausea, vomiting, stomach pain, and diabetic ketoacidosis (DKA). Our Type 1 diabetes guide covers this in more detail.
• Type 2 diabetes: symptoms often take years to develop, and the CDC notes that some people don't notice any symptoms at all. When they do appear, they're usually the gradual, classic symptoms above, sometimes alongside slow-healing wounds or dark patches of skin around the neck, armpits, or groin. Our understanding type 2 diabetes guide covers prevention and management.

High Blood Sugar Without Diabetes

A high reading doesn't automatically mean diabetes. Stress, illness or infection, certain medications (including corticosteroids), a large carbohydrate-heavy meal, and dehydration can all raise glucose temporarily in people who don't have diabetes. An isolated high reading tied to an obvious cause usually isn't a concern on its own, but readings that are repeatedly high, or high without an obvious explanation, are worth discussing with a doctor. Our high blood sugar without diabetes guide covers the common non-diabetic causes in more depth.

Symptoms by Severity

High blood sugar is better understood as a spectrum than a single condition:

• Mild: often no symptoms, or only the subtle early signs above. Found incidentally on a blood test.
• Moderate: the classic symptoms — thirst, frequent urination, fatigue, blurred vision — become noticeable and persistent.
• Severe: symptoms escalate to nausea, vomiting, abdominal pain, confusion, or breathing changes. This range overlaps with diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS), both medical emergencies described below.

There's no single glucose number that marks the line between these stages for everyone — it depends on how quickly glucose rose, how long it's been elevated, and the individual.

When Symptoms May Be Absent

High blood sugar can be present without any symptoms at all, which is one of the main reasons diabetes and prediabetes are diagnosed with blood tests rather than by how someone feels. The CDC notes that type 1 diabetes often has no symptoms in its earliest stage, type 2 diabetes can have few or no symptoms for years, and gestational diabetes usually doesn't cause symptoms either. This is why routine screening matters, particularly for people with risk factors for diabetes, rather than waiting for symptoms to appear.

DKA Warning Signs

Diabetic ketoacidosis (DKA) happens when the body, lacking enough insulin, breaks down fat for energy at a dangerous rate, producing ketones that build up in the blood. It's most common in type 1 diabetes but can occur in type 2 diabetes as well, particularly during severe illness, and it can develop within 24 hours.

Warning signs include:

• Very high blood sugar
• Excessive thirst and frequent urination
• Nausea and vomiting
• Abdominal pain
• Rapid, deep breathing
• A fruity smell on the breath
• Confusion or difficulty concentrating
• Weakness or fatigue

The American Diabetes Association advises checking for ketones when blood glucose is higher than 200 mg/dL (or the level your doctor recommends), and seeking emergency care immediately if a blood ketone reading is 1.6 mmol/L or higher, or if DKA symptoms are present. Our dedicated diabetic ketoacidosis guide covers this emergency in full.

HHS Warning Signs

Hyperosmolar hyperglycemic state (HHS) is a different diabetes emergency, seen mainly in people with type 2 diabetes that isn't well controlled. Unlike DKA, it usually develops more gradually, over days or weeks, and typically involves little or no ketone buildup. According to MedlinePlus, it begins with increased thirst and urination, then progresses to:

• Weakness
• Nausea
• Dry mouth and dry tongue
• Confusion
• Fever
• Seizures
• Coma in severe, untreated cases

Blood sugar in HHS can rise far higher than in DKA — MedlinePlus describes it as sometimes more than ten times a normal level — because the slower onset allows glucose to climb for longer before it's noticed.

When to Seek Emergency Care

Treat any of the following as a medical emergency and seek care immediately rather than waiting to see if it improves:

• Confusion, extreme drowsiness, or loss of consciousness
• Rapid or labored breathing
• A fruity smell on the breath
• Persistent nausea or vomiting, especially with high blood sugar
• Signs of severe dehydration
• A blood ketone reading of 1.6 mmol/L or higher

These are the warning signs of DKA or HHS, and both require hospital treatment — they are not something to manage at home.

How High Blood Sugar Is Diagnosed

Symptoms alone don't diagnose high blood sugar or diabetes — blood tests do. The main tests are:

• Fasting blood glucose: below 100 mg/dL is normal; 100–125 mg/dL suggests prediabetes; 126 mg/dL or higher, confirmed on a repeat test, meets the diagnostic threshold for diabetes. See our normal fasting blood sugar guide for details.
• Random plasma glucose: 200 mg/dL or higher, together with classic symptoms, can support a diabetes diagnosis, though it's typically confirmed with another test.
• HbA1c: below 5.7% is generally normal, 5.7%–6.4% is prediabetes, and 6.5% or higher is the threshold used to diagnose diabetes. See our HbA1c normal range guide for the full breakdown.
• Oral glucose tolerance test (OGTT): used mainly when fasting glucose or HbA1c results are unclear, or for gestational diabetes screening during pregnancy.

Our diabetes blood tests guide compares these tests in more detail, including which one a doctor might choose and why.

High Blood Sugar vs. Diabetes Symptoms

"High blood sugar" and "diabetes" aren't quite the same thing, and the distinction matters for how symptoms are interpreted:

• High blood sugar is a lab finding — a glucose reading above the normal range, at a single point in time.
• Diabetes is a diagnosis, made from a pattern of blood test results (fasting glucose, HbA1c, OGTT, or random glucose with symptoms), not from symptoms alone.
• A single high reading can happen without diabetes, for the non-diabetic reasons described above.
• Diabetes, once diagnosed, is what makes repeated or severe high blood sugar likely — but the symptoms of high blood sugar themselves are the same whether or not someone has a diabetes diagnosis yet.

In short: symptoms can suggest high blood sugar, but only blood tests confirm whether diabetes is the cause. Our diabetes overview guide covers diagnosis, types, and treatment in full.

Sources

• Hyperglycemia (High Blood Glucose) — American Diabetes Association
• DKA (Ketoacidosis) & Ketones — American Diabetes Association
• Diabetes Symptoms — CDC
• High Blood Sugar - Self-Care — MedlinePlus
• Diabetic Hyperglycemic Hyperosmolar Syndrome — MedlinePlus