Chills and shivering: causes, red flags and when to seek help

When the brain resets the body thermostat to a higher setting in response to infection, the body feels cold relative to the new target. Shivering and the sensation of being cold are the means by which it climbs towards that point. Once the temperature rises, sweating and flushing follow, which is why chills often alternate with feeling hot.

Shivering without any infection can happen when you are cold, frightened, exhausted, hungry, or after surgery or anaesthesia. Some medicines, and sudden drops in blood sugar, can also provoke trembling that feels like shivering.

The NHS describes a high temperature as often caused by the body fighting an infection, such as a cold or flu, and as not usually a sign of anything serious. Ordinary viral illness is by far the most frequent reason for chills.

• Colds, flu and COVID-19: chills, aches, tiredness and a raised temperature developing over a day or so.

• Throat, chest or ear infections: shivering with localised pain or cough.

• Urinary tract infection: chills with burning on passing urine, particularly if it has reached the kidneys, with back or flank pain.

• Skin infections such as cellulitis: shivering with a red, hot, tender patch of skin.

• Tonsillitis and dental abscess: chills with a very sore throat or facial swelling.

• Malaria in someone who has recently travelled to an affected area: repeating cycles of shaking chills and fever.

• Low blood sugar, thyroid overactivity, anxiety attacks and some medicines: shivering or trembling without infection.

The NHS makes the point that you may have a high temperature even if a thermometer says your temperature is below 38C, if you feel hot, cold or shivery. It also lists a chest or back that feels hotter than usual, sweating and warm red skin, which may be harder to see on black or brown skin.

It advises using a digital thermometer under the tongue or in the armpit, or a digital ear thermometer, and warns against glass thermometers, forehead strips and some infrared thermometers. If shivering persists with normal readings, think about thyroid disease, anaemia, low blood sugar or anxiety, and mention it at your appointment.

Sepsis is a serious reaction to an infection that the NHS says can be life-threatening and needs urgent hospital treatment. In adults it lists confusion or slurred speech, uncontrollable shivering, muscle pain, difficulty breathing, blue, pale, grey or blotchy skin, lips or tongue, a high or low temperature, and not peeing all day or very little in the past 18 hours.

The NHS says to call 999 or go to A&E if an adult or child aged five or over is breathing very fast, is confused or not making sense, has blue, pale or blotchy skin, has a very high or very low temperature or feels hot or cold to the touch or is shivery, has a rash that does not fade when pressed, or has symptoms you are worried might be sepsis. For babies and children under five the list includes a weak, high-pitched or continuous cry, being hard to wake and not having peed for twelve hours.

Symptoms can be harder to notice in people with dementia, a learning disability, or difficulty communicating, so a change in behaviour with shivering should be taken seriously. A stiff neck, severe headache, or light sensitivity with fever and chills are also reasons for urgent assessment.

You will be asked how quickly the chills began, whether they come in waves, what other symptoms accompany them, who you have been in contact with, and whether you have recently travelled, had a procedure, started a new medicine, or have a cut, bite or catheter. The clinician will check temperature, pulse, blood pressure, breathing rate and oxygen level, look at the skin, throat, ears and chest, and feel the abdomen.

Depending on what is found, a urine dipstick, blood tests for infection markers, swabs for flu or COVID-19, a chest X-ray, or blood cultures may be done. People who appear very unwell are usually assessed in hospital.

The NHS advice for a high temperature in adults is to get lots of rest, drink plenty of fluids so your pee is light yellow and clear, and take paracetamol or ibuprofen if you feel uncomfortable. It advises staying at home and avoiding contact with other people until you no longer have a high temperature.

Layering light covers, rather than piling on heavy blankets, helps avoid overheating once the fever turns. If the temperature is not improving or is getting worse despite treatment at home, the NHS says to ask for an urgent GP appointment or call NHS 111.