Hyperthermia is an abnormally high body temperature. Also called heat illnesses, there are several forms of hyperthermia. Heat cramps are fairly mild, whereas heat exhaustion is more severe. Heatstroke is the most serious form of hyperthermia and can be life-threatening.
Hyperthermia is an abnormally high body temperature — or overheating. It's the opposite of hypothermia, when your body is too cold. Hyperthermia occurs when your body absorbs or generates more heat than it can release. A human’s normal body temperature is about 98.6 degrees Fahrenheit. Any body temperature above 99 or 100 degrees Fahrenheit is too warm.
Hyperthermia is usually the result of overexertion in hot, humid conditions. Most forms of hyperthermia are preventable.
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Hyperthermia isn’t the same as a fever. When you have hyperthermia, your body temperature rises above a certain “set-point” that’s controlled by your hypothalamus (a part of your brain that controls many body functions). But when you have a fever, your hypothalamus actually increases your body’s set-point temperature. This intentional rise in body temperature is your body’s attempt to fight off an illness or infection.
Hyperthermia describes a group of heat illnesses that include (from least to most severe):
Malignant hyperthermia is a genetic condition that can make you susceptible to hyperthermia if you receive a specific combination of sedatives and anesthesia for medical procedures. If you have this disease, your body temperature can spike dangerously during or after surgery. People with malignant hyperthermia might have a reaction the first time they’re exposed to certain medications, but it’s more common after several exposures.
Anyone can get a heat illness, but you’re at especially high risk if you:
Between 2004 and 2018, an average of 702 heat-related deaths occurred in the U.S. each year. Athletes, outdoor laborers, military trainees, infants and the elderly are the most likely to develop hyperthermia.
Hyperthermia occurs when your body takes in more heat than it releases. Sweat is your body’s natural cooling mechanism, but sometimes sweating isn’t sufficient to keep your body at a normal temperature. When this happens, your body temperature can rise. Physical exertion in very hot, humid weather is the most common cause of hyperthermia.
Heat cramps typically cause sudden muscle spasms in your feet, calves, thighs, hands or arms. The cramps might feel painful or tense. Your muscles might be sore after the cramp passes.
Symptoms of heat exhaustion include:
Symptoms of heatstroke can often be similar to those of heat exhaustion, but may also include:
People with heatstroke can develop shock, slip into a coma, experience organ failure or die. If you experience symptoms of heat stroke, seek immediate medical attention.
Healthcare providers diagnose hyperthermia by reviewing your symptoms, performing a physical exam and taking your temperature. They may also order blood or urine tests.
If you experience mild-to-moderate heat illness symptoms, you should:
You can usually treat these conditions at home. In some cases, your healthcare provider may recommend intravenous (IV) fluid replacement through a catheter in your arm.
Heatstroke is a medical emergency that requires treatment in a hospital. If you’re waiting for an ambulance to arrive, try to cool the person as much as possible:
At the hospital, you may receive cooled IV fluids. A healthcare provider may also recommend cold-water lavage. This is a procedure that washes out body cavities with cold water.
In most cases, it’s possible to prevent hyperthermia. You should:
If you must do activities in the heat due to your job or a sport, let your body gradually acclimate to the heat. Start doing light work or exercise about two weeks before you need to do the really hard work. Then, gradually build your body’s ability to withstand high temperatures. If possible, plan to do the work during the early morning hours when it might be cooler.
Most people recover fully from heat illnesses once they rest in a cool environment and replace lost electrolytes. But heatstroke can lead to permanent organ damage or even death.
Contact your doctor if you or someone else:
A note from Cleveland Clinic
Hyperthermia, or heat illness, is an abnormally high body temperature. It’s usually the result of doing too much physical activity in hot, humid weather. Infants, the elderly, athletes and people with strenuous outdoor occupations are at the highest risk for hyperthermia. Heat illnesses can range from mild (heat cramps or heat exhaustion) to severe (heatstroke). People with a heat illness should cool their body temperature right away. Call 911 immediately at the first signs of heatstroke.
Last reviewed by a Cleveland Clinic medical professional on 11/19/2021.
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