Most of us are familiar with the exhaustion that can follow a long day in the sun. While it’s normal to feel hot when the mercury is high, heat intolerance is the sensation of being overly and uncomfortably hot¹.
By any definition, heat intolerance is not cool. Beyond being uncomfortable, it can also be a sign of serious illness, so be sure to check in with your family doctor if you’re a hot mess when everyone around you is cool as cucumbers. And read on to learn more about what causes heat intolerance, and how you can treat it.
What is heat intolerance?
We all get hot from time to time, but heat intolerance is the feeling of being extremely and uncomfortably hot.
Heat intolerance by definition
If your heart rate and core temperature rise faster and earlier than other people in the same environmental conditions, then by definition, you are heat intolerant.²
In objective terms, this means a core temperature of 101.3°F or more, and a heart rate over 150 beats per minute.³ Other signs of heat intolerance include sweating, exhaustion, fatigue, dizziness and vomiting.⁴
How does heat intolerance occur?
When we’re comfortable, we’re probably not thinking about our core temperature, even though thermoreceptors (temperature-sensitive structures connected to the central nervous system, or CNS) are constantly communicating with the brain’s temperature-regulating hypothalamus to keep it stable. When the CNS tells the hypothalamus that we’re too cold, the hypothalamus communicates to the blood vessels, muscles and internal organs, and to the thyroid gland, redirecting blood flow away from skin and towards internal organs, and creating heat through shivering and the release of metabolism-boosting thyroid hormone.⁵
When we’re too hot, the same system tells blood vessels to dilate, sending heat away from the warm core and out towards the cooler peripheral skin to be released. It also directs sweat glands to produce cooling sweat.⁶
Heat intolerance is a sign that this thermoregulatory system is either overwhelmed or that some part of the system is impaired.⁷
What can cause heat intolerance?
Anyone’s thermoregulatory system can become overwhelmed under extreme conditions, such as very hot weather, but some people are more susceptible to thermoregulatory impairment than others. Heat intolerance is associated with lower rates of aerobic fitness and is more likely to affect women than men⁸—but it’s important to remember these are correlations, not causes.
Several medical conditions are also associated with heat intolerance:
Diabetes
Complications of diabetes can lead to damaged nerves and blood vessels. If a person with diabetes has experienced damage in the area of their sweat glands, they’ll have a harder time cooling down in hot weather, particularly if they’re also dehydrated, or if their blood sugars are too high or too low.⁹
Multiple sclerosis
Even a slight increase in temperature can exacerbate MS symptoms, since heat makes it harder for MS patients’ already-damaged CNS to conduct its electrical impulses.¹⁰¹¹ In fact, heat is such an unfortunately reliable trigger, that doctors used to put suspected MS patients into hot baths, looking for worsening neurological symptoms to confirm the diagnosis of MS.¹²
Thyroid conditions
Thyroid hormone is integral to maintaining the body’s metabolism. People whose bodies make too little thyroid hormone—as seen in all conditions with hypothyroidism such as Hashimoto’s disease—tend to run cold. On the other side, hyperthyroidism, usually caused by Graves’ disease, occurs when the thyroid gland makes too much thyroid hormone. This causes the metabolism to work harder, and the body’s temperature to increase.
Heat sensitivity can be a sign of hyperthyroidism, so it’s a good idea to either check your levels at home with a thyroid test kit or, get seen by a doctor if this is something you experience, especially if you also have other symptoms such as anxiety, insomnia, unexplained weight loss, fast heart beats, vision problems, muscle weakness, irregular periods or a goiter.¹³
Potential triggers of heat intolerance
Warm temperatures and overexertion are among the obvious triggers to an episode of heat intolerance, but certain foods and medications can also increase sensitivity to heat:
Caffeine
Caffeine is a stimulant that can increase your heart rate, your metabolic rate and raise your temperature.¹⁴¹⁵
Medications
Some common over-the-counter antihistamines and decongestants can increase heat sensitivity, as can certain blood pressure medications, antidepressants, antipsychotics and some treatments for overactive bladder.¹⁶
Several prescription drugs can also make you more sensitive to the sun and can lead to heatstroke if you’re not careful. These include certain antibiotics, diabetes medications, pain relievers, hormones, cancer drugs and skin treatments.¹⁷
How can I treat heat intolerance?
If you’ve been diagnosed with diabetes, MS or hypo- or hyper-thyroidism, or suspect you might have one of these conditions, you should discuss your concerns with your doctor. Same goes if you rely on a medication that is known to cause heat intolerance.
No matter the cause of your heat intolerance, these lifestyle habits will reduce your chances of getting dangerously hot¹⁸:
- Stick to cool environments, seeking air conditioning and shade as much as possible
- Drink plenty of water
- Wear breezy, loose-fitting clothes in natural fabrics
- Take your medications on schedule
- Save exercise and strenuous activities for cooler parts of the day or for air-conditioned indoor environments
- Take cool (but not cold!) showers, suck on ice cubes, and apply cool cloths to wrists and neck as needed
It’s also good practice to consider humidity as well as heat before planning your time outside. High humidity makes it harder for sweat to evaporate, an essential step in the body’s natural cooling process. Forecasts based on the heat index—which factors the way humidity makes the air feel hotter—offer a more accurate idea of what it will really be like once you get outside.
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