Severe heat intolerance causes: what to know

Your body normally cools itself through sweating and increased blood flow to the skin. Heat intolerance develops when cooling cannot keep pace with the surroundings or when the body produces more heat than usual. Heat exhaustion can cause heavy sweating, weakness, headache, nausea, dizziness, and a rising body temperature. Hyperthyroidism can make the body more active, causing heat intolerance alongside a fast heartbeat, tremor, anxiety, weight change, or difficulty sleeping. Higher body weight can make heat loss harder. Dehydration and some medicines can also reduce tolerance, while nerve or skin conditions may interfere with sweating.

Heat intolerance does not identify one diagnosis by itself. A clinician considers when it began, whether sweating is reduced or excessive, your medicines, other symptoms, and exposure to heat.

Move to a cool, shaded or air-conditioned place, loosen extra clothing, and sip cool fluids if you are awake and able to drink. Use a cool damp cloth or a fan, and avoid strenuous activity during the hottest part of the day. Do not stop prescribed medicines without medical advice. Arrange a medical review if the problem is new, worsening, recurrent, or limiting ordinary activities; take a list of medicines and note triggers.

Seek emergency help for confusion, fainting, a seizure, severe weakness, trouble breathing, chest pain, or inability to drink. Urgent assessment is also needed when someone exposed to heat remains very unwell or has a very high body temperature despite cooling. These signs can indicate serious heat illness rather than simple sensitivity to warm weather.

Ask whether your symptoms suggest thyroid disease, heat exhaustion, dehydration, or a medicine effect; whether you need blood tests; and how to exercise, work, or travel safely in hot weather.