Many illnesses cause dehydration, but the most common culprits are gastrointestinal infections, diabetes, fever-producing illnesses, kidney disorders, and adrenal conditions. Some medications also trigger significant fluid loss. The underlying mechanism varies, but the result is the same: your body loses more water and electrolytes than it takes in, and normal functions start to break down.
Stomach and Intestinal Infections
Gastroenteritis, often called the stomach flu, is the single most common illness behind acute dehydration. Viruses like rotavirus and norovirus are the predominant causes in developed countries, though bacterial infections from E. coli, salmonella, and shigella also play a major role. These pathogens attack the lining of your gut, triggering diarrhea and vomiting that can drain fluid remarkably fast.
The volume of fluid lost through diarrhea alone can range from near-normal levels to over 200 mL per kilogram of body weight per day. For context, that means a 150-pound adult with severe cholera-type diarrhea could lose several liters of fluid daily through stool alone. Different pathogens also strip away different amounts of sodium, a critical electrolyte. Rotavirus causes moderate sodium losses, while cholera drives losses three to four times higher. This is why severe diarrheal illness doesn’t just make you thirsty; it throws off your entire electrolyte balance.
Children and older adults are especially vulnerable. Young children have smaller fluid reserves relative to their body size, so they reach dangerous levels of dehydration faster. In infants, warning signs include fewer wet diapers than usual, dry mouth or lips, no tears when crying, unusual sleepiness or fussiness, and a sunken soft spot (fontanelle) on the top of the head. A sunken fontanelle is a reliable visual indicator that a baby needs fluids.
Diabetes and Blood Sugar Problems
Uncontrolled diabetes is one of the most significant non-infectious causes of dehydration. When blood sugar rises too high, the kidneys can’t reabsorb all that excess glucose, so it spills into the urine. That glucose pulls water along with it, a process called osmotic diuresis. The result is excessive urination, which then triggers intense thirst and excessive drinking, the classic signs of uncontrolled diabetes.
The math illustrates how dramatic this effect can be. A person with blood sugar around 360 mg/dL and normal kidney filtration will filter roughly 2,000 units of glucose per day, but the kidneys can only reabsorb about half of that. The remainder pulls enough water to produce an extra 3 or more liters of urine daily. This fluid loss also carries sodium and potassium out of the body, creating electrolyte deficits on top of the water deficit. Diabetic ketoacidosis, a serious complication of type 1 diabetes, produces especially severe dehydration through this mechanism and requires emergency treatment.
Diabetes Insipidus
Despite the similar name, diabetes insipidus has nothing to do with blood sugar. This condition involves a hormone called antidiuretic hormone (ADH), which tells your kidneys to hold onto water. When your brain doesn’t produce enough ADH, or when your kidneys don’t respond to it properly, the kidneys release enormous volumes of dilute urine, sometimes many liters per day. You feel relentlessly thirsty and can become dehydrated quickly if you can’t keep up with the fluid loss.
There are two main forms. In the central type, the brain produces too little ADH, often due to surgery, head injury, or a tumor near the pituitary gland. In the kidney-based type, the kidneys simply ignore the ADH signal. Both lead to the same problem: the body can’t properly balance its fluid levels, and dehydration becomes a constant risk rather than an occasional one.
Adrenal Insufficiency (Addison’s Disease)
The adrenal glands sit on top of your kidneys and produce hormones that regulate salt and water balance. In Addison’s disease, these glands stop producing enough of those hormones. One hormone in particular, aldosterone, is responsible for telling the kidneys to hold onto sodium. Without it, sodium pours out through urine, sweat, saliva, and the digestive tract, dragging water with it.
This creates a dangerous combination: low sodium, high potassium, and progressive dehydration. If untreated, the ongoing salt and water loss leads to severe dehydration, low blood pressure, and eventually circulatory collapse. People with Addison’s disease typically need daily hormone replacement to maintain normal fluid balance. During illness or physical stress, they often need to increase their dose to prevent a dehydration crisis.
Fever and Respiratory Illness
Any illness that raises your body temperature increases your fluid needs. Fever speeds up the evaporation of water through your skin, a type of invisible water loss you don’t notice the way you notice sweating. The standard clinical estimate is that fluid loss through the skin increases by about 10% for every degree Celsius above 38°C (100.4°F). A sustained high fever of 40°C (104°F), for example, would raise your baseline skin water loss by roughly 20%.
Respiratory infections add another layer. When you breathe faster, as happens with pneumonia, bronchitis, or even a bad cold, you exhale more water vapor with each breath. Combine that with fever, reduced appetite, and the general misery that makes you less likely to drink, and respiratory illnesses can produce meaningful dehydration even without any vomiting or diarrhea.
Medications That Cause Fluid Loss
Several classes of medication deliberately increase urine output or cause fluid loss as a side effect. Diuretics, commonly prescribed for high blood pressure and heart failure, work by forcing the kidneys to excrete more sodium and water. That’s their intended function, but the line between therapeutic fluid removal and dehydration can be thin, especially in hot weather or during illness.
A newer class of diabetes drugs, SGLT2 inhibitors, also causes notable fluid loss. These medications lower blood sugar by blocking glucose reabsorption in the kidneys, which means glucose and water leave the body through urine, much like what happens in uncontrolled diabetes but in a controlled way. Volume depletion is recognized as a class-wide effect of these drugs. Most cases are mild and manageable with adequate hydration, but the risk increases if you’re also taking a diuretic or not drinking enough fluids.
How to Gauge Severity
Dehydration is categorized by the percentage of body weight lost as fluid. In infants, mild dehydration corresponds to up to 5% weight loss, moderate is 6 to 10%, and severe is 10 to 15%. Older children reach concerning thresholds sooner: mild is up to 3%, moderate around 6%, and severe around 9%. Adults follow a similar pattern, though exact weight-based thresholds are harder to track because adults are less frequently weighed during illness.
Practical signs are more useful for most people than percentages. Early dehydration shows up as thirst, darker urine, dry mouth, and fatigue. Moderate dehydration adds dizziness when standing, a rapid heartbeat, and noticeably reduced urine output. Severe dehydration brings confusion, very rapid heart rate, sunken eyes, and skin that stays “tented” when pinched rather than snapping back immediately. In infants, that sunken soft spot is one of the clearest visible warning signs.
The speed of onset matters as much as the cause. A stomach virus can push a small child from well-hydrated to severely dehydrated in under 24 hours, while chronic conditions like Addison’s disease or diabetes insipidus produce a slower, grinding fluid deficit that worsens over days or weeks if untreated. Recognizing the underlying illness driving the dehydration is the key to treating it effectively, because replacing fluids without addressing the cause means the losses will simply continue.