Heat intolerance: causes, warning signs and practical steps

Heat intolerance is feeling unusually uncomfortable or unwell in warm conditions when your body has difficulty cooling itself.

You may feel flushed, sweaty, weak, dizzy, irritable, tired, or unable to carry on with your usual activity. Some people notice a racing heartbeat, headache, nausea, muscle cramps, or trouble concentrating. Symptoms can appear indoors, outdoors, during exercise, or after a hot shower.

Move to a cool place and seek urgent medical help if someone is confused, faints, has a seizure, cannot drink, has very hot skin, or becomes rapidly more unwell. These signs can occur when overheating is severe and need prompt assessment.

Get medical advice the same day for ongoing vomiting, marked weakness, or symptoms that do not settle after cooling down and taking small sips of fluid.

Heat intolerance can occur with heat exhaustion, hyperthyroidism, or obesity. These causes are listed alphabetically rather than by how often they explain the symptom.

Heat exhaustion can develop after heat exposure and fluid loss. Hyperthyroidism can make body processes run faster and may bring sweating, tremor, weight change, or a fast heartbeat. Extra body insulation can make heat dissipation more difficult for people with obesity.

A clinician will ask about the setting, duration, fluids, activity, sleep, medical conditions, and medicines. They may check temperature, pulse, blood pressure, hydration, and signs of illness.

When symptoms persist outside clear heat exposure, blood tests may be used to look for thyroid problems or other explanations. Bring a list of medicines, supplements, and a record of when symptoms occur.

Heat-related illness is treated by cooling, resting, and replacing fluid when safe to do so; severe symptoms need urgent care. If testing points to an overactive thyroid, treatment aims to control thyroid hormone activity.

A personalised plan may also address medicines, activity timing, clothing, hydration, and conditions that make cooling harder. Do not change prescribed medicines without professional advice.

Schedule demanding activity for cooler parts of the day, take breaks in air-conditioned or shaded spaces, and wear loose, breathable clothing. Carry fluids and drink regularly according to your health needs, especially if you are active or sweating.

Acclimatise gradually when returning to outdoor work or exercise after a break. People with heart, kidney, or fluid-restriction plans should ask their clinician what drinking plan is appropriate for them.

Record sweating changes, thirst, dizziness, palpitations, tremor, headache, nausea, cramps, and urine becoming dark or infrequent. These details help distinguish dehydration, heat illness, and non-heat-related causes.

Repeated heat intolerance can limit daily activity and may signal an underlying condition or a medication issue. Addressing the trigger can reduce future episodes and lower the chance of progressing to heat-related illness during exposure.