Cold chills happen when your brain raises its internal temperature target and your body scrambles to catch up, triggering muscle contractions, goosebumps, and that unmistakable shivery feeling. Infections are the most common cause, but chills can also stem from low blood sugar, thyroid problems, anemia, anxiety, and even certain medications. Understanding what’s behind the sensation helps you figure out whether your chills are a normal immune response or something that needs attention.
How Your Body Creates Chills
Your brain’s thermostat sits in a region called the hypothalamus. Under normal conditions, it keeps your core temperature hovering around 98.6°F (37°C), though anywhere between 97°F and 99°F is typical. When something signals the hypothalamus to raise that set point, your current body temperature suddenly registers as “too cold,” even though it hasn’t actually dropped. Your brain responds the same way it would if you stepped outside in winter: it constricts blood vessels near your skin to trap heat inside, triggers tiny muscles at the base of each hair follicle (giving you goosebumps), and activates rapid, involuntary muscle contractions. That shivering is your body’s furnace. It generates heat until your actual temperature climbs to match the new, higher set point.
This is why chills so often come before a fever rather than during one. You feel coldest in the gap between where your temperature is and where your brain thinks it should be. Once the fever peaks and the set point resets back to normal, the process reverses: you suddenly feel too hot, blood vessels dilate, and you start sweating to dump the excess heat.
Infections: The Most Common Trigger
When bacteria, viruses, or fungi invade your body, immune cells release signaling molecules (primarily interleukin-1, tumor necrosis factor, and interferon) that act on the hypothalamus. These molecules trigger the production of a chemical called prostaglandin E2, which is the direct signal that pushes your thermostat upward. The result is the classic sequence: chills, then fever, then sweats as the fever breaks. Bacterial toxins, particularly from certain types of bacteria, are among the most potent triggers of this cycle.
Common infections that cause chills include the flu, COVID-19, urinary tract infections, pneumonia, and strep throat. Virtually any infection can do it, though some produce more dramatic chills than others. Malaria, for example, is notorious for causing cyclical waves of intense, shaking chills.
Shaking Chills and Bacteremia
There’s an important distinction between mild chills and rigors, which are severe, whole-body shaking episodes where you feel cold to the bone even under heavy blankets. A 2024 meta-analysis in BMC Medicine found that shaking chills are a highly specific predictor of bacteremia, meaning bacteria have entered the bloodstream. The specificity was 87%, meaning that when shaking chills are absent, there’s a strong chance the person doesn’t have bacteremia. The sensitivity was only 37%, though, so plenty of bacteremia cases don’t produce rigors at all. Still, an episode of true shaking chills, especially with high fever, is a signal worth taking seriously.
Low Blood Sugar
When blood glucose drops below about 70 mg/dL, your autonomic nervous system fires up a stress response. This triggers shakiness, sweating, a racing heart, and often a chills-like sensation that can be hard to distinguish from infection-related chills. The key difference: low blood sugar chills come on suddenly, typically don’t involve fever, and are accompanied by hunger, lightheadedness, or difficulty concentrating. Eating or drinking something with fast-acting sugar resolves the episode within minutes. People with diabetes are most at risk, but anyone who hasn’t eaten for an extended period or who exercises intensely on an empty stomach can experience it.
Thyroid Problems
Your thyroid gland controls your basal metabolic rate, which is essentially how much heat your body generates at rest. When thyroid hormone levels drop too low (hypothyroidism), your internal furnace runs cooler. The result is a persistent feeling of being cold, with a lower tolerance for cool environments and a tendency toward chills that healthy people wouldn’t experience at the same temperature.
Thyroid hormones drive heat production through multiple pathways, including activating specialized fat tissue that burns calories purely to produce warmth and regulating how muscles use energy. Without adequate thyroid hormone, both of these systems underperform. Unlike infection-related chills, thyroid-related cold intolerance tends to be chronic and gradual rather than sudden. Other hallmarks include fatigue, weight gain, dry skin, and sluggish thinking. A simple blood test can confirm the diagnosis.
Iron-Deficiency Anemia
Iron deficiency impairs temperature regulation through two competing problems. First, low iron disrupts thyroid function, reducing heat production at its source. Second, when your blood carries less oxygen (because iron is essential for building oxygen-carrying red blood cells), your body faces a trade-off: it needs blood flow to the skin to stay warm, but it also needs that limited oxygen-carrying blood directed to vital organs. The organs win, blood gets diverted away from the skin, and you feel cold. Research confirms that anemia is a central factor in the inability of iron-deficient people to regulate their temperature when exposed to even mild cold.
Anxiety and the Stress Response
Panic attacks and intense anxiety can produce chills without any change in body temperature. The mechanism is your fight-or-flight system activating thermoregulatory pathways that normally respond to physical threats. Your body may trigger piloerection (goosebumps), constrict blood vessels in the skin, and activate sweating all at once, creating a confusing mix of feeling cold, clammy, and flushed simultaneously.
Chills and hot flushes are recognized symptoms in the diagnostic criteria for panic attacks. A systematic review published in Frontiers in Psychiatry noted that the connection between anxiety disorders and the body’s temperature control system is only beginning to be understood, but the physical experience is very real. If you notice chills that come on during stressful moments, resolve once you calm down, and never produce an actual fever, anxiety is a likely explanation.
Medications That Cause Chills
A surprisingly long list of medications can trigger fever and chills as a side effect, a phenomenon called drug fever. Chills from medication can look identical to chills from an infection, which makes diagnosis tricky. Research shows that 36 to 51 percent of drug fever cases involve shaking chills, the same kind typically associated with serious bacterial infections.
Drug categories most commonly linked to fever and chills include:
- Antibiotics: penicillin-family drugs, cephalosporins, vancomycin, and several others
- Chemotherapy agents: many cancer drugs trigger chills as part of an immune or inflammatory reaction
- Heart and blood pressure medications: certain blood thinners, diuretics, and rhythm-control drugs
- Anti-seizure medications: carbamazepine and phenytoin
- Pain relievers: some NSAIDs, including ibuprofen and naproxen, can paradoxically cause fever
- Immunosuppressants: drugs used after organ transplants or for autoimmune conditions
The telltale sign of drug fever is that chills and fever begin after starting a new medication and resolve when it’s stopped. If you develop unexplained chills shortly after beginning a prescription, it’s worth flagging to your prescriber.
Other Common Causes
Cold exposure is the most obvious and benign cause. When your skin temperature drops, the hypothalamus activates shivering to generate heat without any change to the internal set point. This is straightforward thermoregulation, not a sign of illness.
Dehydration can contribute to chills because your body becomes less efficient at regulating temperature when fluid levels are low. Intense exercise sometimes causes post-workout chills as your body rapidly shifts from heat-dumping mode back to normal, particularly in cool or windy conditions. Emotional responses, like hearing a moving piece of music or experiencing a strong memory, can trigger brief chills through nervous system pathways unrelated to temperature regulation.
When Chills Signal Something Serious
Most chills are part of a routine immune response and resolve on their own. But certain combinations of symptoms suggest something more urgent is happening. Chills with a fever above 103°F (39.4°C) in an adult typically indicate a significant infection. Shaking rigors, where your whole body trembles uncontrollably, are associated with bacteria in the bloodstream and warrant prompt evaluation.
Chills alongside confusion, rapid breathing, a heart rate that feels unusually fast, or skin that looks mottled or feels cold and clammy can point toward sepsis, a life-threatening response to infection that requires emergency treatment. Chills paired with sudden difficulty breathing, chest pain, or bluish discoloration of the lips or fingertips also need immediate attention, as these suggest the heart or lungs are involved. In children, any rectal temperature of 100.4°F (38°C) or higher in an infant under three months old is considered an emergency regardless of other symptoms.