Fever due to dehydration: what should you know?

A fever is a regulated rise in body temperature, commonly linked to infection. Dehydration means the body has lost more fluid than it has replaced. Fluid loss reduces sweating and limits the body's ability to cool itself, so overheating can occur, especially after exertion or exposure to hot weather. Heat exhaustion can therefore include thirst, weakness, dizziness, headache, nausea, heavy sweating and a raised temperature.

Vomiting or diarrhoea from gastroenteritis can remove fluid while also causing fever. A urinary tract infection may produce fever with reduced drinking or fluid loss. Dark urine, passing urine less often, dry mouth, tiredness and light-headedness support dehydration, but they do not identify its cause. A thermometer reading, recent fluid loss, medicines, medical conditions and other symptoms help a clinician work out what is happening.

Move to a cool place, loosen extra clothing and rest. Take frequent small sips of water. If vomiting or diarrhoea has caused fluid loss, an oral rehydration solution can help replace water and salts; follow the product directions. Continue drinking as you are able and choose light foods when appetite returns. Avoid alcohol, strenuous activity and very sugary drinks while recovering.

Record the temperature, how often you pass urine, and whether fluids stay down. Do not force large amounts quickly if you feel nauseated. Children, older adults, pregnant people and people with kidney, heart or other conditions may need individual advice about fluid replacement.

Seek urgent medical help for confusion, fainting, severe weakness, trouble breathing, a seizure, inability to drink, repeated vomiting, or very little or no urine. These can signal serious dehydration, heat illness or another urgent condition. Also seek prompt assessment for a fever that is high or persistent, worsening symptoms, severe abdominal pain, stiff neck, a new widespread rash, or blood in vomit or stool.

Do not assume dehydration explains a fever after travel, with urinary symptoms, or when there is no clear fluid loss. A clinician may need to check for infection and assess hydration, particularly in a young child or frail adult.

Ask what may be causing the temperature, which fluids are suitable for you, and whether you need an examination or tests. Tell the clinician when the fever began, its highest reading, recent vomiting or diarrhoea, urine changes, heat exposure, medicines and any condition that affects fluid intake or output.