Chills happen when your body rapidly contracts and relaxes muscles to generate heat, usually because your brain’s internal thermostat has been reset higher than your actual body temperature. The most common trigger is an infection like the flu, but chills can also result from low blood sugar, anxiety, dehydration, thyroid problems, or even certain medications. Understanding what else is going on alongside your chills is the fastest way to narrow down the cause.
How Your Body Creates Chills
Your brain has a built-in thermostat, and when something disrupts it, your body reacts the same way it would if you walked into a freezer. During an infection, immune cells release signaling molecules that travel to the brain and raise its target temperature. Your brain then registers your current body temperature as “too cold” and kicks off a set of responses: shivering to produce heat, narrowing blood vessels near your skin to reduce heat loss, and behavioral urges like curling up or reaching for a blanket.
This is why you can feel freezing cold even when you’re actually running a fever. Your body is working hard to close the gap between where your temperature is and where your brain now thinks it should be. Once your temperature catches up to the new set point, the chills typically stop, though you may then feel hot or sweaty as the fever breaks.
Infections: The Most Common Cause
If your chills came on suddenly alongside body aches, fatigue, or a sore throat, an infection is the most likely explanation. The flu is a classic culprit. Unlike a common cold, which builds gradually, flu symptoms tend to hit hard and fast, with fever, chills, muscle aches, headaches, and exhaustion arriving together. A fever is generally defined as a temperature of 100.4°F (38°C) or higher, though chills can appear before a thermometer picks up any change.
Respiratory infections like pneumonia and bronchitis also cause chills, especially when accompanied by a deep cough or difficulty breathing. Urinary tract infections can trigger chills too, particularly when the infection has spread beyond the bladder to the kidneys. Gastrointestinal infections from food poisoning often pair chills with nausea, vomiting, or diarrhea. In each case, the underlying process is the same: your immune system detects a pathogen and resets your thermostat upward.
Low Blood Sugar
Dropping blood sugar can produce chills, shaking, and sweating that feel surprisingly similar to being sick. For people with diabetes, this typically happens when blood sugar falls below 70 mg/dL. For people without diabetes, symptoms usually appear below 55 mg/dL. Your body responds to low blood sugar by flooding your system with stress hormones, triggering a fight-or-flight reaction that causes trembling, weakness, sweating, and a cold or clammy feeling.
If you haven’t eaten in several hours, skipped a meal, or exercised intensely without fueling up, low blood sugar is worth considering. Eating or drinking something with fast-acting carbohydrates (juice, glucose tablets, a piece of fruit) usually resolves the chills within 15 to 20 minutes.
Anxiety and Panic Attacks
Chills are a recognized symptom of panic attacks. During a panic episode, adrenaline levels can spike to two and a half times their normal level or more. That surge puts your body on high alert: your heart races, your breathing becomes fast and shallow, your blood sugar jumps, and your muscles tense. Chills or hot flashes are part of this cascade, as your nervous system redirects blood flow and your skin temperature fluctuates rapidly.
These chills can feel identical to infection-related chills, which often makes them more alarming and feeds the panic cycle. The key difference is context. If your chills arrive alongside a racing heart, shortness of breath, a sense of dread, and no other signs of illness, anxiety is a strong possibility. The chills typically pass within 20 to 30 minutes as the adrenaline wears off.
Dehydration and Physical Exertion
Feeling chills after a hard workout or a long stretch in the heat often points to dehydration. Your body relies on adequate fluid levels to regulate temperature through sweating and blood flow. When you’re dehydrated, these cooling and warming systems lose efficiency, and your body can struggle to maintain a stable temperature even in a warm environment.
Dehydration also reduces blood volume, which drops blood pressure and limits circulation to your skin and extremities. That poor circulation makes you feel cold. In more serious cases, dehydration during exercise or heat exposure can progress to heat exhaustion, where chills develop as the body begins to overheat and its cooling mechanisms start to fail. If chills after exertion come with dizziness, confusion, or you stop sweating despite the heat, that’s a sign your body is in real trouble.
Thyroid Problems
If you’re someone who always feels cold when others are comfortable, and chills are a recurring theme rather than a sudden episode, your thyroid may be involved. Thyroid hormones are major regulators of metabolism and heat production. When the thyroid is underactive (hypothyroidism), your metabolic rate slows down, and your body simply doesn’t generate enough heat to keep you warm. Cold intolerance is one of the hallmark symptoms.
Other signs of an underactive thyroid include unexplained fatigue, weight gain, dry skin, constipation, and feeling sluggish. Unlike infection-related chills that come and go, thyroid-related cold sensitivity tends to be persistent. A simple blood test can confirm whether your thyroid hormone levels are low.
Medications and Cancer Treatments
Certain medications can cause chills as a side effect, sometimes severe enough to cause full-body shaking called rigors. This is particularly common with cancer treatments. Biologic therapies like interferons and certain antibody-based drugs frequently trigger flu-like symptoms, including chills, fever, and body aches. Several chemotherapy drugs are also associated with chills as part of a broader flu-like reaction.
If you recently started a new medication or received an infusion and developed chills shortly afterward, the drug itself may be the cause. Your treatment team can often manage this with pre-medications or adjustments to how the drug is delivered.
When Chills Signal Something Serious
Most chills are your body doing exactly what it’s designed to do in response to a minor infection or stressor. But chills combined with certain other symptoms can indicate a medical emergency, particularly sepsis. Sepsis occurs when your body’s response to an infection spirals out of control and begins damaging your own tissues and organs.
Warning signs that chills may be part of something more dangerous include:
- Confusion or difficulty thinking clearly
- Rapid heart rate or fast breathing
- A temperature above 104°F (40°C) or below 95°F (35°C)
- Extreme pain or discomfort
- Warm, clammy, or sweaty skin
- Significant drop in how much you’re urinating
Sepsis is a life-threatening emergency that requires immediate treatment. A high heart rate, confusion, or rapid breathing alongside chills are early signals that something beyond a routine infection may be happening.
Managing Chills at Home
For garden-variety chills from a cold, flu, or mild illness, the basics work well. Layer up with blankets to ease the discomfort of shivering, but don’t pile on so much that you overheat once your fever breaks. Stay hydrated, since fever and sweating both increase fluid loss. Over-the-counter pain relievers like acetaminophen or ibuprofen can help bring down a fever and reduce the chills that come with it.
Monitor your temperature periodically. A moderate fever in an otherwise healthy adult is not inherently dangerous and is actually a sign your immune system is fighting effectively. But a temperature that climbs above 104°F, drops below 95°F, or persists for more than three days warrants professional evaluation. The same applies if your chills keep returning without an obvious explanation, or if they’re accompanied by symptoms that don’t fit a typical viral illness.