Chills happen when your brain decides your body needs to be warmer and triggers rapid, small muscle contractions to generate heat. The causes range from the obvious (fighting off an infection) to the surprising (anxiety, low iron, or hormonal shifts). Chills can show up with or without a fever, and the underlying reason determines whether they’re a minor nuisance or something worth investigating.
How Your Body Creates Chills
A region at the front of your hypothalamus acts as your internal thermostat. When it senses that your core temperature is too low, or when an infection tricks it into raising your temperature set point, it sends signals down through the brainstem to your spinal cord. Those signals activate your skeletal muscles in a rapid, involuntary pattern: shivering. Each tiny contraction converts energy into heat, warming your blood and raising your body temperature.
This same circuit also fires during fever. When your immune system fights an infection, it releases chemical messengers that act on the hypothalamus and essentially reset the thermostat higher. Your body then perceives its current normal temperature as “too cold” and triggers chills to close the gap. That’s why you can feel freezing under a blanket even though your skin is hot to the touch.
Infections Are the Most Common Cause
If your chills came on suddenly and you’re feeling unwell, an infection is the most likely explanation. The flu is a classic trigger, but bacterial infections like pneumonia, urinary tract infections, and listeria can all cause shaking chills. Parasitic infections such as giardiasis do too. Not every infection produces a fever right away, so you can experience chills before a thermometer confirms anything is wrong, or even without ever developing a measurable fever.
For reference, a low-grade fever in adults runs from 99.1 to 100.4°F, while a high-grade fever starts at 102.4°F. If your temperature is normal but you’re still getting chills, that doesn’t rule out infection. It may mean your immune system is responding early, or that a milder illness is at play.
Anxiety and the Adrenaline Response
Strong emotions, especially fear, shock, or anxiety, can trigger chills without any illness at all. When your brain perceives a threat (real or imagined), the amygdala signals the hypothalamus, which activates your adrenal glands. Those glands flood your bloodstream with adrenaline as part of the fight-or-flight response. Among other effects, adrenaline causes the tiny muscles around your hair follicles to contract, producing goosebumps and that shivery, chilled sensation.
This type of chill is usually short-lived and passes once the stressful moment fades. But if you’re dealing with chronic anxiety or panic attacks, you might notice chills recurring regularly, sometimes without an obvious emotional trigger. The sensation is real and physical, not imagined. Your nervous system is genuinely activating the same pathways it would use if you were standing in the cold.
Low Thyroid Function
Thyroid hormones are major regulators of metabolism and heat production. When your thyroid is underactive (hypothyroidism), your metabolic rate drops, and your body simply doesn’t generate as much warmth. Cold intolerance is one of the hallmark symptoms. People with hypothyroidism often describe feeling chilled in rooms that everyone else finds comfortable, needing extra layers, or noticing that their hands and feet are perpetually cold.
If your chills are persistent rather than sudden, and they come with fatigue, weight gain, dry skin, or sluggishness, an underactive thyroid is worth considering. A simple blood test can check your thyroid hormone levels.
Iron Deficiency and Anemia
Iron plays a less obvious role in temperature regulation. When your iron levels are low, your body has trouble producing hemoglobin, the protein in red blood cells that carries oxygen. With less oxygen reaching your tissues, your overall energy production drops. But iron deficiency also impairs thyroid metabolism and disrupts heat generation in brown fat tissue, the specialized fat your body uses specifically to produce warmth. The result is a persistent feeling of being cold that doesn’t fully resolve by adding layers.
Iron deficiency anemia is especially common in women with heavy periods, people on restrictive diets, and those with conditions that reduce iron absorption. Other signs include unusual fatigue, pale skin, brittle nails, and shortness of breath during mild activity.
Hormonal Shifts During Perimenopause and Menopause
Most people associate menopause with hot flashes, but cold flashes are just as real. Fluctuating hormone levels during midlife make the brain’s internal thermostat more sensitive and less stable. Your body can swing from feeling overheated to suddenly chilled within minutes. Some people experience cold flashes on their own, while others get them as the tail end of a hot flash, when the body overcorrects after a surge of heat and drops temperature too quickly.
These episodes tend to cluster during perimenopause, when hormone levels are at their most erratic, and often ease once the body adjusts to its new hormonal baseline.
Medications That Cause Chills
Certain medications can trigger intense chills, sometimes called rigors, as a side effect. This is most common with drugs delivered by IV infusion, particularly antibody-based therapies used in cancer treatment and immune suppression. One widely used cancer drug triggers rigors in 13% to 33% of patients during infusion, while another immune-suppressing antibody causes rigors in roughly 55% of patients. Some antifungal medications are also known to cause shaking chills.
If your chills started around the same time as a new medication, whether it’s an infusion, an oral drug, or even an over-the-counter supplement, that timing is worth mentioning to your doctor.
When Chills Signal Something Serious
In most cases, chills are your body doing exactly what it’s designed to do. But in rare situations, chills are an early warning sign of sepsis, a dangerous overreaction by the immune system to an infection. Sepsis is a medical emergency, and recognizing it early can be lifesaving.
The combination of symptoms to watch for includes chills or shaking along with a fast heart rate, rapid breathing, confusion or disorientation, extreme weakness, and fever (or unusually low body temperature). Warm, clammy skin and a sudden drop in how much you’re urinating are also red flags. Any combination of these symptoms, especially if they’re worsening quickly, warrants an immediate trip to the emergency room. Sepsis progresses fast, and early treatment dramatically improves outcomes.
Narrowing Down Your Cause
The pattern of your chills tells you a lot. Sudden chills with body aches, sore throat, or congestion point toward an infection. Chills that come and go with stressful moments suggest an adrenaline-driven response. A constant, low-level feeling of being cold that doesn’t match the room temperature hints at thyroid or iron issues. Chills that follow a hot flash pattern fit hormonal changes.
Pay attention to what else is happening in your body. Fever, even a mild one, makes infection the most likely explanation. Fatigue and weight changes suggest a metabolic cause. If the chills resolve on their own within a day or two and you feel fine otherwise, your body was likely fighting off something minor. If they persist for more than a few days, recur without explanation, or come with any of the serious warning signs above, that’s a signal worth following up on.