Dehydration and body temperature: what to know

Your body normally cools itself partly through sweating and blood flow near the skin. When you have lost fluid, there may be less circulating volume and less sweat available, so you can feel unusually hot, weak, dizzy or faint, especially in a warm environment. This is different from a fever, in which the body raises its internal temperature in response to an illness or another trigger.

A raised temperature together with thirst, dry mouth, dark urine, passing urine less often, headache or tiredness can occur when an illness is causing both fever and dehydration. Vomiting or diarrhoea can speed up fluid loss. Heat exhaustion is another possible concern when heavy sweating, weakness, nausea or headache follows heat or exertion.

Move to a cool place, loosen extra clothing and rest. Sip water regularly rather than drinking a large amount quickly. If vomiting or diarrhoea has caused the fluid loss, an oral rehydration solution may help replace water and salts; follow the product instructions. Continue with small, frequent sips if your stomach feels unsettled.

Avoid alcohol and strenuous activity while you feel dehydrated. Keep track of whether you can drink and pass urine. Dark urine can be a useful sign of concentrated urine, although urine colour can also change because of medicines, foods or other health problems. Do not use cold baths or ice to manage a very high temperature, because shivering can increase heat production.

Seek urgent help if the person is confused, difficult to wake, fainting, having a seizure, struggling to breathe, unable to keep fluids down, or becoming very hot after heat exposure. These signs can indicate severe dehydration, heat illness or another emergency.

Arrange prompt medical advice for a persistent fever with worsening dehydration symptoms, very little or no urine, blood in vomit or stool, severe abdominal pain, or symptoms in a baby, older adult, pregnant person or someone with a significant medical condition. A clinician can assess the cause and decide whether fluids, tests or other treatment are needed.

Ask what may be causing the temperature change, whether your symptoms fit dehydration or heat illness, which fluids are suitable for you, and which warning signs should prompt same-day or emergency care. Mention any vomiting, diarrhoea, new medicine, urinary symptoms or recent heat exposure.