The most common symptoms of high blood sugar are urinating more often (especially at night), unusual thirst, tiredness, blurred vision, hunger, and slow-healing cuts or frequent infections. They usually appear once glucose is persistently above about 180 mg/dL (10 mmol/L). Vomiting, confusion, drowsiness, rapid breathing or fruity-smelling breath alongside high glucose are emergency signs.
Why high blood sugar causes symptoms
Two mechanisms explain almost every symptom on this page.
First, the kidney threshold. The kidneys filter glucose out of the blood and normally reabsorb all of it. Above roughly 180 mg/dL (10 mmol/L), they cannot keep up, and glucose passes into the urine. Glucose pulls water with it, so the body loses fluid, and everything that follows — urination, thirst, dry mouth, dehydration, headache — stems from that.
Second, fuel shortage inside cells. In insulin resistance or insulin deficiency, glucose cannot enter muscle and fat cells efficiently. The cells behave as if starved: the body signals hunger, breaks down fat and muscle for energy, and the person feels tired and loses weight even while eating more.
The symptoms, roughly in order of appearance
Urinating more often
Usually the first sign, and often noticed first at night (nocturia). Once glucose spills into urine, the kidneys produce more of it. Waking two or three times a night to urinate, when that is new, is worth a test.
Thirst and a dry mouth
The body's response to fluid loss. The thirst can be hard to satisfy, because drinking more water simply produces more urine while glucose remains high. Sugary drinks make it worse.
Tiredness
Caused by the combination of dehydration, disturbed sleep from night-time urination, and cells short of fuel. It is a heavy, persistent fatigue rather than sleepiness.
Blurred vision
Glucose enters the lens of the eye and draws fluid in, changing its shape and focus. The blurring typically fluctuates with glucose and settles once levels stabilise — which is why opticians advise waiting a few weeks after a diabetes diagnosis before ordering new glasses. It is different from the permanent damage of diabetic retinopathy (see how high blood sugar affects the body), which develops over years and is painless and initially symptomless.
Increased hunger
Cells cannot access glucose, so appetite signals stay switched on. Eating more raises glucose further without relieving the hunger.
Unintended weight loss
When glucose is lost in urine and fat and muscle are broken down for fuel, weight falls. Losing several pounds without trying, particularly with thirst and urination, is a strong signal of significantly raised glucose.
Slow-healing cuts and frequent infections
High glucose impairs white blood cell function and circulation. Cuts heal slowly, and infections — particularly thrush (yeast) in the mouth, skin folds or genitals, urinary tract infections, and skin infections — recur.
Tingling or numbness in the feet or hands
A later sign, reflecting early nerve damage from sustained high glucose. It often starts as pins and needles or a burning feeling in the toes.
Darkened skin in body folds
Acanthosis nigricans — velvety, darkened skin on the back of the neck, in the armpits or groin — is a sign of high insulin levels — see signs of insulin resistance — rather than high glucose, and often precedes diabetes.
Headache, irritability and difficulty concentrating
Dehydration and glucose swings both contribute.
Symptoms in people who already have diabetes
For someone being treated, symptoms returning usually mean glucose has drifted above target: a missed dose, an illness, a change in diet or activity, or a treatment plan that needs updating. The same symptoms can also appear during a rapid fall in glucose after a period of high readings, as the body readjusts.
When symptoms are an emergency
Seek emergency care if high blood sugar is accompanied by any of: vomiting or inability to keep fluids down · abdominal pain · deep, rapid breathing · breath that smells fruity or like nail-polish remover · confusion, drowsiness or difficulty staying awake · a glucose meter reading above 400 mg/dL (22 mmol/L) or "HI". These can indicate diabetic ketoacidosis or hyperosmolar hyperglycemic state, both of which can be fatal without treatment.
When to see a doctor
Make an appointment if you have any combination of increased urination, thirst, fatigue, blurred vision or unexplained weight loss — or recurrent infections or slow-healing wounds — even if each on its own seems minor. An A1C or fasting glucose test will show whether glucose is the explanation. If you have risk factors for diabetes and no symptoms, ask to be screened anyway; the absence of symptoms is the norm in early disease.
Frequently asked questions
How high does blood sugar have to be before you notice symptoms?
Usually above about 180 mg/dL (10 mmol/L), the point at which glucose starts passing into the urine. Below that, most people feel nothing, which is why prediabetes and early diabetes are usually found by testing.
Can you have high blood sugar and feel fine?
Yes. Glucose that rises slowly is easy to adapt to, and many people with an A1C of 7 or 8% report no symptoms. Feeling fine is not evidence that glucose is normal.
Are the symptoms different in women?
The core symptoms are the same. Women more often notice recurrent vaginal thrush and urinary tract infections, because glucose in the urine feeds yeast and bacteria.
Is itchy skin a sign of high blood sugar?
It can be. Dehydration dries the skin, and high glucose encourages fungal infections in skin folds. Persistent itching without an obvious cause is a reason to test.
References
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Type 2 Diabetes (2023). https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/type-2-diabetes
- NIDDK. Diabetes Tests & Diagnosis (2023). https://www.niddk.nih.gov/health-information/diabetes/overview/tests-diagnosis
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