Chills happen when your brain decides your body needs to be warmer, triggering rapid muscle contractions that generate heat. The most common reason is a fever from an infection, but chills can also result from cold exposure, low blood sugar, anxiety, or other conditions that activate your body’s temperature-control system. Understanding what’s behind your chills depends on what else is happening in your body at the same time.
How Your Body Creates Chills
A small region at the front of your brain called the hypothalamus acts as your internal thermostat. It constantly receives temperature signals from sensors in your skin and throughout your body, then sends commands to either cool you down or warm you up. When it detects that your core temperature is too low, or when it raises its target temperature during a fever, it triggers a chain of signals that travel down through your brainstem and spinal cord to your muscles.
Those muscles then contract and relax rapidly in short bursts. That’s the shivering you feel. It’s the same basic mechanism whether you’re standing outside in winter or lying in bed with the flu. Your body is essentially using your muscles as a furnace, converting energy into heat through rapid, involuntary movement. At the same time, blood vessels near your skin constrict to keep warm blood closer to your vital organs, which is why your hands and feet often feel cold when you have chills.
Fever: The Most Common Cause
When your immune system detects an infection, it releases chemical signals that reach the hypothalamus and effectively raise the thermostat’s set point. Your normal body temperature of around 98.6°F suddenly registers as “too cold” to your brain, even though nothing about the environment has changed. Your body then does exactly what it would do if you walked into a freezer: it shivers, constricts blood vessels, and drives you to seek warmth by curling up under blankets.
This is why chills often come in waves at the beginning of a fever. Your body is actively working to push your temperature up to that new, higher set point. Once your temperature reaches the target, the chills typically stop, and you may actually feel hot instead. A fever is generally defined as an oral temperature of 100°F (37.8°C) or higher. If your temperature reaches 103°F (39.4°C) or above, that warrants medical attention.
Common infections that cause chills include the flu, COVID-19, urinary tract infections, pneumonia, and strep throat. Chills with fever can also follow certain vaccinations, which trigger a similar immune response without an actual infection.
Cold Exposure and Hypothermia
If you’re simply cold, your chills are a straightforward survival mechanism. Shivering can start even when your core temperature is still technically normal, in the range of 95 to 98.6°F. Your skin sensors detect the cold environment and alert the hypothalamus before your core temperature drops significantly.
If your core temperature falls below 95°F, you’ve entered hypothermia. Shivering intensifies as your body fights to generate heat. But once core temperature drops to around 90°F, shivering actually stops, not because things are improving, but because your muscles are too exhausted and your metabolism is shutting down. Loss of shivering in a cold environment is a dangerous sign, not a reassuring one.
Low Blood Sugar
If you have diabetes or sometimes skip meals, chills and shakiness can signal that your blood sugar has dropped too low. Hypoglycemia typically triggers symptoms when blood glucose falls below 70 mg/dL. Your body responds by releasing adrenaline, which causes trembling, sweating, a racing heart, and a cold or clammy feeling that can easily be mistaken for chills from illness. Eating or drinking something with sugar usually resolves symptoms within 15 to 20 minutes.
Anxiety and Adrenaline
Chills that strike during a stressful moment, a panic attack, or after a frightening experience have a different origin than fever-related chills, but the sensation can feel identical. When your fight-or-flight system activates, adrenaline floods your bloodstream. This diverts blood away from your skin and toward your muscles and vital organs, preparing you to respond to a threat. The result is pale, cold skin and sometimes visible shaking or trembling.
People with panic disorder often describe sudden waves of chills or cold flashes as one of their most noticeable symptoms. Trauma survivors and people with PTSD can experience the same thing when a memory or trigger activates their stress response. These chills tend to come on suddenly, last minutes to an hour, and resolve as the adrenaline surge fades. They’re not dangerous on their own, but they can be distressing, especially if you don’t recognize the cause.
Other Medical Causes
Several conditions can produce chills without an obvious explanation:
- Thyroid problems. An underactive thyroid slows your metabolism, making it harder for your body to generate heat. People with hypothyroidism often feel cold when others are comfortable and may experience frequent chills.
- Anemia. When your blood carries fewer red blood cells or less oxygen, your circulation becomes less efficient at keeping you warm. Persistent chills alongside fatigue and pale skin can point to anemia.
- Medication side effects. Some drugs can cause chills as a side effect, particularly medications that affect serotonin levels or blood pressure. Chills can also be a sign of serotonin syndrome, a rare but serious reaction to certain combinations of medications that affect brain chemistry.
- Hormonal shifts. Fluctuations in estrogen during menopause or menstruation can temporarily disrupt the hypothalamus, causing sudden cold flashes or chills alongside the more commonly discussed hot flashes.
Chills With No Fever
If you’re getting chills but your thermometer reads normal, that narrows the possibilities. Cold exposure, low blood sugar, anxiety, anemia, thyroid dysfunction, and hormonal changes all produce chills without raising your temperature. Sleep deprivation and extreme fatigue can also make you more sensitive to cold and trigger shivering at temperatures that wouldn’t normally bother you.
Pay attention to when your chills happen. Chills that strike during stressful moments point toward adrenaline. Chills that come with shakiness and lightheadedness before meals suggest blood sugar. Chills that persist day after day, especially with fatigue or weight changes, may reflect a thyroid or blood-related issue worth investigating with a simple blood test.
When Chills Signal Something Serious
Most chills are harmless and short-lived. But intense, uncontrollable shaking (sometimes called rigors) combined with a high fever can indicate a severe infection. Sepsis, the body’s extreme and life-threatening response to an infection, frequently causes shaking chills alongside confusion, rapid breathing, or a feeling that something is seriously wrong. Sepsis is a medical emergency that requires immediate treatment to prevent organ damage.
Chills that come with a stiff neck and sensitivity to light may signal meningitis. Chills paired with severe back or side pain and painful urination can indicate a kidney infection. And chills following surgery, a dental procedure, or a catheter insertion could mean bacteria have entered the bloodstream. In any of these scenarios, the combination of symptoms matters more than the chills alone.