High Blood Sugar Symptoms: Causes, Signs, and Taking Control – Diabetes Self-Management

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If you’ve had diabetes for any length of time at all, you’ve probably seen lists of the signs and symptoms of high blood sugar dozens of times. Doctors and diabetes educators hand them out. Hundreds of websites reprint them. Most diabetes books list them. You likely know some of the items on the list by heart: thirst, frequent urination, blurry vision, slow healing of cuts, and more.

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But have you ever stopped to wonder why these symptoms occur? How does high blood sugar cause frequent urination, make your vision go blurry, or cause all of those other things to happen? Here are some answers to explain what’s going on in your body when you have high blood sugar.

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Setting the stage for high blood sugar

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High blood sugar (called hyperglycemia by medical professionals) is the defining characteristic of all types of diabetes. 

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It happens when the body can no longer maintain normal blood glucose levels, either because the pancreas isn’t producing enough insulin or because the body’s cells have become resistant to insulin. As a result, glucose accumulates in the bloodstream rather than moving into the cells.

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— Learn More About Blood Glucose Management >>

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What is high blood sugar?

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Blood glucose is commonly considered too high if it is higher than 130 mg/dl before a meal or higher than 180 mg/dl two hours after the first bite of a meal. However, most of the signs and symptoms of high blood glucose don’t appear until the blood glucose level is higher than 250 mg/dl. Some of the symptoms have a rapid onset, while others require a long period of high blood glucose to set in.

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Learn more about normal blood sugar levels: Diabetes Blood Sugar Chart | Blood Glucose Chart.

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It’s important to note that individuals differ in their sensitivity to the effects of high blood glucose: Some people feel symptoms more quickly or more strongly than others. But each sign or symptom has a biological underpinning, or a specific cause behind the effect.

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Hyperglycemia can be acute or chronic. Acute hyperglycemia lasts only briefly and is often the result of a high-carbohydrate meal, a missed dose of medicine, stress, or illness. 

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Chronic hyperglycemia, on the other hand, is a state of long-term elevated blood glucose. It is often the result of undiagnosed diabetes or of an inadequate diabetes treatment regimen. 

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Chronic hyperglycemia is arguably the more dangerous of the two, as long-term elevated blood glucose has a toxic effect on the body’s tissues. In fact, some of the signs of high blood glucose are actually the aftermath of cellular damage caused by high blood glucose.

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Signs and symptoms of high blood sugar are often what lead people with undiagnosed diabetes to visit their doctors and, consequently, get diagnosed. But signs and symptoms of high blood sugar can also occur after diabetes is diagnosed and treatment has been started. It is a signal that your diabetes has slipped out of control.

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The classic symptoms of high blood sugar are polyuria, polydipsia, and polyphagia

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In plain English, that means excessive urination, excessive thirst, and excessive hunger. Any doctor who hears this trio of complaints will reach for a blood glucose meter. But often, the person experiencing these symptoms doesn’t notice them right away. 

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This is partly because they often creep up on a person in a gradual fashion, and partly because the signs and symptoms of high blood glucose aren’t well known among people who don’t have diabetes — or don’t know they have diabetes.

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Here’s what’s behind these classic three symptoms of high blood sugar:

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Excessive urination

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Polyuria is the result of a runaway biological and chemical chain reaction that feeds on itself. It starts in the blood, where high glucose concentrations osmotically pull intracellular fluid into the bloodstream. 

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This is the body’s attempt to equalize the concentration of glucose in the blood with the concentration in the cells. By diluting the blood with intracellular fluid, the body brings the glucose concentration of the blood closer to normal. Initially, this increases the fluid volume of the blood while dehydrating the cells.

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Meanwhile, in the kidneys, a related dysfunction is brewing. Normally, the kidneys serve as filters, removing waste products and returning cleansed fluid back to the body. The return of the cleansed fluid — or reabsorption of fluid — takes place in the renal tubules, the internal structure of the million or so filtering nephrons in each kidney. 

However, when the glucose concentration of the fluid entering the nephrons exceeds 250 mg/dl, the reabsorption capacity of the renal tubules is blocked, triggering what is known as osmotic diuresis — a discharge of large amounts of urine. Until the glucose levels are normalized, the renal tubules can’t regain the ability to absorb fluids.

So a dual chain reaction has begun. The cells are pumping water into the bloodstream, and the kidneys, unable to reabsorb this fluid during filtering, are uncontrollably flushing water from the body. The result is runaway urination.

To meet the clinical definition of polyuria, urine output for an adult must exceed 2.5 liters per day (normal urine output is 1.5 liters per day); however, extremely high blood glucose can lead to outputs of 15 liters (about 4 gallons) per day, a fluid loss similar to that seen in cholera victims. In rare cases, polyuria can top out at 20–25 liters per day, about half the body’s total fluid volume.

The dehydrating effect of polyuria is a key player in many of the other signs and symptoms of high blood glucose, including polydipsia.

Excessive thirst

Polydipsia is a response to the dehydrating effects of polyuria; it’s the body’s attempt to rehydrate itself. Thirst signals in the brain are triggered by osmoreceptors, specialized cells in the hypothalamus that monitor blood concentration. 

When dehydration increases plasma osmolality (the blood’s solute level), osmoreceptors activate the thirst response to encourage fluid intake.

The relationship between excessive urination and excessive thirst is often misinterpreted by people experiencing high blood glucose, especially before diabetes is diagnosed. Many people mistakenly believe that drinking more causes excessive urination, overlooking the fact that high blood glucose triggers both thirst and urination independently. So they are quick to discount the excessive urination, thinking “Well, I’ve been drinking a lot lately…â€

Complicating matters, many people reach for a soda when they’re thirsty, and most sodas contain both some form of sugar and caffeine. The sugar raises the blood glucose level even higher, and while caffeine has a mild diuretic effect, it can contribute to dehydration, particularly in those who are not regular caffeine consumers.

Excessive hunger

Excessive hunger is primarily driven by cells’ inability to access glucose due to low or ineffective insulin. In type 1 diabetes, hunger results from an absolute lack of insulin. 

In type 2 diabetes, hunger often stems from insulin resistance, where cells do not respond effectively to insulin, even if some insulin is present. In either case, the amount of insulin in the blood is insufficient to move glucose molecules from the bloodstream into the cells, where they can be used as fuel for cellular processes.

If cells aren’t able to gain access to glucose, they send hunger signals through hormones like ghrelin (which increases appetite) and leptin (which promotes satiety). In diabetes, especially type 2, leptin resistance can impair satiety signals, contributing to persistent hunger. 

The cells have no way of knowing that they’re starving in the land of plenty: The glucose they need exists in abundance nearby in the bloodstream, but the relative deficiency of insulin makes it inaccessible. 

This can lead to a cycle where eating in response to hunger further raises blood glucose levels. Choosing low-glycemic, high-fiber foods can help manage hunger without exacerbating hyperglycemia.

Weight loss

Even though you may be eating like crazy when your blood glucose is chronically high, you will still lose weight, and there are three reasons for this:

First, excessive urination (polyuria) leads to dehydration and fluid loss, contributing to a drop in body weight. This fluid loss often explains the rapid, early weight loss seen with new-onset diabetes, particularly type 1.

Second, when insulin is insufficient, the body enters a catabolic state, breaking down both fat and muscle to meet energy demands. This can lead to significant and unhealthy weight loss.

Third, excess glucose is excreted in the urine (glycosuria), leading to calorie loss and weight reduction. This can happen when blood glucose levels exceed the renal threshold (typically around 180-200 mg/dl).

Some individuals with type 1 diabetes, especially adolescents and young adults, may intentionally reduce or omit insulin doses to induce weight loss — a dangerous eating disorder known as diabulimia. This practice can lead to severe complications such as diabetic ketoacidosis (DKA), nerve damage, and organ failure.

Unexplained weight loss, especially when accompanied by increased hunger, thirst, or fatigue, can be a warning sign of uncontrolled diabetes. Seeking medical attention promptly can help diagnose and manage the condition before serious complications develop.

Infections

Your body’s cells aren’t the only inhabitants of the microscopic world that eat glucose: Many bacteria and yeasts do, too.

Urinary tract infections (UTIs) and yeast infections can occur in both men and women, but they are much more common among women, and they’re particularly common among women with diabetes. In fact, women with diabetes are three times more likely to have asymptomatic bacteria in their urine than women who don’t have diabetes.

Both bacteria and yeast thrive in warm, dark, moist places and feed on glucose. Chronic yeast infections are common in women with chronically high blood glucose. The cause may simply be a case of supply and demand: More glucose gives yeast more opportunities.

UTIs, on the other hand, are quite a bit more complicated, and high blood glucose contributes to them in several ways beyond providing glucose-rich urine for the bacteria to grow in. 

Long-term elevated blood glucose can lead to complications, including neuropathy (nerve damage), which affects various body systems. Neuropathy can affect the bladder‘s ability to contract properly, causing incomplete emptying. This leaves behind residual pools of static urine in the bladder — the perfect growth culture for bacteria.

In addition, elevated glucose reduces blood circulation, which in turn reduces the ability of infection-fighting leukocytes (white blood cells) to get where they are needed in a timely manner in sufficient numbers to fight off infections. High blood glucose also reduces phagocytosis, the process leukocytes use to ingest bacteria.

Slow healing of cuts and wounds

High blood sugar greatly slows the healing of skin and soft-tissue infections because neutrophils, the most common type of leukocyte in the immune system’s arsenal, are particularly vulnerable to high levels of glucose.

Elevated blood sugar levels impair neutrophil function by affecting their ability to adhere to the endothelium (the inner lining of blood vessels), disrupting chemotaxis (the process that directs neutrophils to injury or infection sites), and hindering phagocytosis (the ingestion of harmful bacteria and debris).

Adequate oxygen supply is essential for wound healing. High blood glucose can lead to peripheral vascular disease (narrowing of blood vessels), reducing blood flow and oxygen delivery to tissues. Additionally, diabetic neuropathy can result in loss of sensation, making individuals less aware of injuries, which may delay treatment and exacerbate wound severity.

The slow healing of wounds sets the stage for some of the grimmest diabetes complications. Minor wounds can advance to cellulitis, which is a serious infection. Cellulitis can advance to tissue necrosis, or tissue death. 

Tissue necrosis can then extend from the soft tissue into the bone, which is what often leads to amputation: According to the Centers for Disease Control and Prevention (CDC), in 2010, about 73,000 non-traumatic lower-limb amputations were performed in adults aged 20 years or older with diagnosed diabetes in the United States.

Dry, itchy skin

A common but often vexing side effect of high blood glucose is dry, itchy skin. This condition can be attributed to several factors:

Dehydration from excessive urination:

Elevated blood glucose levels can lead to increased urination, causing the body to lose fluids. This dehydration can result in dry skin, as the skin cells lose moisture.

Poor blood circulation:

High blood glucose can cause changes in small blood vessels, leading to reduced circulation. This decreased blood flow can result in dry, itchy skin, particularly in the lower legs and feet.

Nerve damage (neuropathy):

Diabetes can lead to neuropathy, which may impair the function of sweat glands. Since sweating helps keep skin moist, reduced sweating can contribute to dryness and itching.

Diabetic dermopathy:

Unique to people with diabetes, diabetic dermopathy appears as light brown, scaly patches, often on the shins. These patches are caused by changes in small blood vessels and are generally harmless, requiring no treatment. However, they serve as a visible sign of diabetes-related skin changes.

It’s important to note that dry, irritated, or itchy skin is more susceptible to infection. People with diabetes may have a reduced ability to fight off infections, making skin care essential.

Maintaining proper skin hydration through regular use of moisturizers, managing blood glucose levels effectively, and monitoring skin for any changes can help mitigate these issues.

Blurred vision

Speaking of dryness, blurry vision from acute high blood glucose is also a result of the dehydrating effect of excessive urination. As you will recall, when the concentration of glucose in the blood is high, the body attempts to dilute the blood by pulling fluid from cells into the bloodstream. 

This effect happens throughout the body, including in the cells of the eyes. Elevated blood sugar leads to fluid shifts, causing the lens to swell and alter its shape, impairing the eye’s focusing ability. 

When the lens of the eye gets dried out, it becomes temporarily warped, and the eye loses its ability to focus properly. This effect is typically temporary and resolves as blood glucose levels normalize.

Chronic high blood sugar, on the other hand, can lead to retinopathy, or damage to the back of the eye that can affect vision and ultimately even lead to blindness. The prevalence of diabetic retinopathy at the time of type 2 diabetes diagnosis varies, with studies reporting rates between 6.9% and 17.2%. 

This is because in most cases, people with type 2 diabetes have elevated blood glucose levels for an extended period before their diabetes is diagnosed. 

This is also why the American Diabetes Association recommends that people with type 2 diabetes have an initial dilated and comprehensive eye exam shortly after diagnosis to detect any retinal changes early and to establish a baseline for ongoing eye health monitoring.

Headaches and difficulty concentrating

Acute hyperglycemia can cause headaches and difficulty concentrating due to cellular energy deprivation. In hyperglycemia, cells may be unable to utilize glucose effectively because of insulin resistance or deficiency, leading to energy deficits that affect brain function. 

This can manifest in symptoms similar to polyphagia — in which starving cells send out hunger signals because they can’t access the glucose circulating in the blood.

Your brain is the biggest glucose hog around. If you don’t believe it, consider that while the brain represents about 2% of your body weight, it devours around 20% of the glucose you consume. 

Maintaining stable blood glucose levels is crucial for optimal cognitive function, as both hyperglycemia and hypoglycemia (low blood sugar) can impair concentration and memory. And when brain cells have difficulty getting the fuel they need, they function poorly. This can cause problems with thinking, reasoning, and remembering, difficulty staying focused on tasks, and headaches.

Chronic high blood glucose can also lead to headaches, but by a different route. These headaches are often related to various types of nerve damage. Examples include occipital neuralgia, which involves irritation or injury to the occipital nerves, causing piercing or throbbing pain in the upper neck, back of the head, or behind the ears. 

Diabetic mononeuropathy can also affect cranial or peripheral nerves, leading to sudden weakness or pain that may contribute to headaches.

While this section focuses on hyperglycemia, it’s important to note that hypoglycemia can also cause headaches and cognitive impairments. Rapid drops in blood glucose trigger hormonal responses that constrict blood vessels in the brain, leading to headaches.

Fatigue

When your blood glucose is high, your body is neither storing nor utilizing glucose properly. You’re not burning energy efficiently, nor are your cells getting the fuel they need when they need it. 

The overall effect is a lowering of physical energy on the cellular level. If you are also suffering from a lack of proper sleep due to excessive urination during the night, you will feel even more tired.

In fact, nearly all the signs and symptoms of high blood glucose can contribute to making you feel fatigued, even exhausted. Exhausted people often resort to eating high-carbohydrate snacks for a boost of quick energy, and this, of course, raises the blood glucose level more, making the problem worse rather than better.

Chronic constipation or chronic diarrhea

Both constipation and diarrhea can be caused by high blood glucose, acting on different sections of the bowel. When high blood glucose affects the small intestine, the result is often diarrhea; when it affects the large intestine, the result is often constipation.

The two intestines have very different jobs, which helps to explain how high blood glucose can affect them differently. The job of the small intestine is to absorb nutrients from digested food, and the job of the large intestine is to absorb water from indigestible waste material.

When neuropathy from elevated blood glucose affects the enteric nerves — the gut’s command and control system — in the small intestine, the result can be dysfunctions in motility, leading to delayed emptying into the large intestine. 

This causes the stagnation of fluids in the small intestine, allowing bacterial overgrowth that causes bloating, abdominal pain, and diarrhea. Chronic diarrhea is present in up to 5% of individuals with diabetes, while constipation is seen in up to 25%. 

Enteric neuropathy, in addition to laying the groundwork for bacterial overgrowth, can in itself be a painful chronic condition similar to the foot pain suffered by people with peripheral neuropathy.

In a similar way, nerve damage can slow the movement of waste through the large intestine. Slow-moving waste is essentially dehydrated — or dried out too much — by the large intestine, resulting in constipation.

Additionally, some diabetes medications, such as metformin, are known to cause diarrhea. Constipation can also be caused or made worse by the dehydrating effects of excessive urination and by some medicines, especially narcotic painkillers, some antidepressants, and the calcium channel blocker class of high blood pressure medicines.

Erectile dysfunction

Erectile dysfunction, or difficulty maintaining an erection, is a common side effect of high blood glucose, and studies indicate that approximately 52.5% of men with diabetes experience ED, with rates of 37.5% in type 1 diabetes and 66.3% in type 2 diabetes.

A healthy erection depends on proper nerve function, adequate blood flow, balanced hormones, and sexual stimulation. The mechanics of it are anything but simple. For an erection to occur, a number of complicated hydraulic events have to happen that cause the penis to fill with blood and then keep the blood trapped inside the penis for a time.

The “action center†of the penis is the corpora cavernosa, a pair of chambers of spongy tissue on either side of the shaft. To form an erection, nerve signals cause the blood vessels that supply the corpora cavernosa to dilate, so that blood flow to the chambers increases. 

As the spongy tissue engorges with blood, it collides with an outer sheath of elastic tissue that encases the chambers. This temporarily chokes off the veins that drain the blood back out of the penis, allowing the erection to stabilize.

High blood sugar can adversely affect erections in three ways: hormonal, vascular, and neurologic.

Hormonal: 

Elevated blood glucose can impair the production of nitric oxide, a vasodilator essential for initiating the chain reaction that relaxes blood vessels and allows the corpora cavernosa to fill with blood.

Vascular: 

Diabetes-related damage to blood vessels may reduce the ability of arteries supplying the penis to dilate properly.

Neurologic: 

Nerve damage resulting from high blood glucose can affect both sensation and the nerve signaling necessary for an erection.

Making matters worse, there are a number of common medicines that can contribute to or worsen erectile dysfunction. Many high blood pressure pills, especially those in the families of beta blockers and diuretics, and some antidepressants, can interfere with vasodilation (or the widening of blood vessels). 

Since both high blood pressure and depression frequently accompany diabetes, it’s common for men with diabetes to be on one or more of these medicines. Lifestyle, too, can increase the risk of erectile dysfunction. For instance, smoking is known to narrow the blood vessels.

High blood pressure can also contribute to erectile dysfunction, as can a build-up of low-density lipoprotein (LDL) cholesterol in the corpora cavernosa supply arteries. How’s that for motivation to take your statin?

The good news is that the drugs Viagra (generic name sildenafil), Cialis (tadalafil), Stendra (avanafil), and Levitra (vardenafil) can all help counteract the loss of nitric oxide, helping the blood vessels dilate and increasing blood flow to the penis. 

However, these medications may be less effective in men with diabetes. Their method of action is on the hormonal and vascular side of the equation, while the cause of erectile dysfunction among men with diabetes is often multifactorial, involving nerve damage as well.

Irritability

The mood-altering effects of high blood glucose are well known to the loved ones of persons with diabetes and well documented by research. 

Acute hyperglycemia has been shown to impair cognitive performance and alter mood states, contributing to increased irritability and decreased mental efficiency. Chronic hyperglycemia is also associated with a higher risk of depression and cognitive decline.

The biological nuts and bolts behind the emotional response aren’t fully understood. Theories explaining the mood-altering effects of high blood sugar range from the straightforward — the brain’s dependence on a steady supply of glucose for energy, causing rapid mood changes when glucose levels fluctuate — to more complex ideas involving nerve conduction velocity in the brain or intricate relationships between hormones and proteins.

But when you consider all the other biological manifestations of high blood sugar — the three polys, infections, slow healing of injuries, itchy skin, blurred vision, headaches, fatigue, and gastrointestinal distress — is it any wonder we might be irritable?

Getting high blood glucose into control

While the causes that lie behind the symptoms of high blood glucose are fascinating, the real value of symptoms is in the message they carry: Blood glucose is too high. 

The symptoms are warning signs, and we ignore them at our peril. High blood sugar does more than trigger biological and chemical processes that make you feel crummy; over time, high blood glucose causes permanent damage to the body.

But you have it in your power to make the symptoms go away, and to keep them from coming back, by keeping your high blood sugar in control. And you’re not alone in this task. Talk to your doctor. Make time to see a diabetes educator. Ask your family members for their support in helping you to eat right and keep active. Use the tools available to you, such as your blood glucose meter, to see whether your numbers are in target range most of the time. If they are not, talk to your medical team about adjusting your therapy. And keep reading Diabetes Self-Management!

Want to learn more about dealing with high blood sugar? Read “Managing Hyperglycemia,†“Strike the Spike II: Dealing With High Blood Glucose After Meals†and “What is a Normal Blood Glucose Level?“

En Español: Síntomas de un Nivel Alto de Azúcar en Sangre: Causas, Síntomas y Tratamiento

Originally Published January 11, 2013