HHS, or hyperosmolar hyperglycemic state, is a life-threatening diabetes emergency where blood sugar climbs above 600 mg/dL and causes severe dehydration. Unlike diabetic ketoacidosis (DKA), which produces dangerous acid buildup, HHS develops because the body still makes enough insulin to prevent that acid crisis but not enough to keep blood sugar under control. It’s most common in people with type 2 diabetes and carries a higher mortality rate than DKA.
How HHS Develops in the Body
HHS starts with a combination of too little insulin and a surge of stress hormones like glucagon, cortisol, and adrenaline. Without enough insulin to move sugar into cells, glucose accumulates in the blood. At first, the kidneys compensate by filtering out the excess sugar into urine, dragging large amounts of water along with it. This is called osmotic diuresis, and it’s why extreme thirst and frequent urination are early warning signs.
As this water loss continues over days or even weeks, blood volume drops. With less blood flowing through the kidneys, they lose their ability to flush out glucose efficiently. Blood sugar then spirals higher, pulling even more water out of cells and into the bloodstream. The result is dangerously concentrated blood (high osmolality) and profound dehydration that can affect every organ, especially the brain. Full normalization of fluid balance and blood chemistry can take up to 72 hours even with hospital treatment.
What Triggers an HHS Episode
Infections are the single most common trigger, responsible for roughly 50% to 60% of all HHS cases. Respiratory infections like pneumonia, urinary tract infections, and gastrointestinal infections top the list. The body’s stress response to infection drives up those counterregulatory hormones that push blood sugar higher.
Cardiovascular events, including heart attacks and strokes, can also set off HHS through the same stress hormone surge. Certain medications are another frequent culprit, particularly in older adults managing multiple conditions. Thiazide diuretics (commonly prescribed for blood pressure), beta-blockers, glucocorticoids, and some atypical antipsychotics can all interfere with blood sugar regulation enough to tip someone into HHS. Sometimes the trigger is simply that someone has stopped taking their diabetes medication or hasn’t been diagnosed with diabetes yet.
Symptoms and Warning Signs
HHS typically develops gradually over several days to weeks, which is part of what makes it dangerous. Early symptoms mirror severe dehydration: intense thirst, dry mouth, frequent urination, and fatigue. Because it builds slowly, these signs are easy to dismiss, especially in older adults who may already have reduced thirst sensation.
What distinguishes HHS from other diabetes emergencies is the severity of neurological symptoms. As blood becomes more concentrated, the brain loses water to the surrounding fluid, leading to confusion, lethargy, and disorientation. In advanced cases, HHS can cause focal neurological deficits (weakness on one side of the body, vision changes), seizures, and coma. Notably, HHS does not produce the fruity-smelling breath or deep, labored breathing patterns seen in DKA, because ketone acid levels stay relatively low.
How HHS Differs From DKA
Both HHS and DKA are hyperglycemic emergencies, but they follow different metabolic paths. In DKA, the body has virtually no insulin available, so it breaks down fat for energy and produces ketone acids as a byproduct. This leads to a dangerous drop in blood pH below 7.3. DKA is more common in type 1 diabetes and tends to develop quickly, over hours to a couple of days, with nausea, abdominal pain, and that characteristic acetone breath.
In HHS, enough circulating insulin remains to prevent significant ketone production, so blood pH typically stays above 7.3. The trade-off is that without the acute sickness of acidosis, HHS can smolder unnoticed for much longer. By the time someone reaches the hospital, blood sugar in HHS is often far higher (above 600 mg/dL, sometimes exceeding 1,000) and dehydration is more severe. About one-third of cases show some overlap between the two conditions, with features of both HHS and DKA present at the same time.
Who Is Most at Risk
HHS occurs most often in older adults with type 2 diabetes, particularly those over 65. Mortality data from the U.S. shows that adults 65 and older have the highest death rate from hyperglycemic crises at roughly 4.7 per 100,000 people, followed by those aged 45 to 64 at about 2.7 per 100,000. Between 1999 and 2019, total U.S. deaths from hyperglycemic crises (including both HHS and DKA) nearly doubled, rising from about 3,300 to over 6,000 annually.
People living in higher-poverty areas face roughly 50% greater mortality from these emergencies compared to those in lower-poverty areas. Other risk factors include limited access to healthcare, living alone (where early symptoms go unnoticed), kidney disease that impairs the body’s ability to clear excess glucose, and cognitive impairment that makes self-managing diabetes more difficult. Though less common, HHS can also occur in children and young adults with either type 1 or type 2 diabetes, usually triggered by an acute illness.
How HHS Is Treated
HHS is treated in a hospital, often in an intensive care unit, and the priority is replacing lost fluids. Rehydration alone begins lowering blood sugar by restoring kidney function and allowing glucose to be filtered out again. The goal is to replace about half of the estimated fluid loss in the first 12 hours and the rest over the following 12 hours. This has to happen carefully: dropping blood concentration too fast can cause dangerous fluid shifts, particularly swelling in the brain.
Insulin is not always started immediately. Current guidelines recommend beginning with fluids first and adding low-dose insulin only once blood sugar stops falling on its own, or if significant ketone levels are present. When insulin is used, the dose is typically lower than what’s given for DKA. Throughout treatment, potassium levels are monitored closely because rehydration and insulin both pull potassium into cells, and a sudden drop can cause heart rhythm problems.
Recovery from HHS is slower than from DKA. Full normalization of blood chemistry and fluid balance often takes up to 72 hours. The underlying trigger, whether it’s an infection, a medication issue, or an undiagnosed condition, also needs to be identified and treated. After recovery, the focus shifts to adjusting diabetes management to prevent a recurrence, since having one episode of HHS significantly raises the likelihood of another.