The experience of suddenly feeling hot, followed by a sensation of being cold, indicates that the body’s internal thermostat is experiencing dysregulation. The body constantly works to maintain a stable core temperature. When this balance is disrupted by infection, hormonal shifts, or stress, the body employs rapid physiological responses to correct it. These corrections cause the noticeable shifts between feeling hot and cold. This article explores the specific systems and external factors that lead to this phenomenon, including immune, hormonal, and nervous system responses.
How the Body Regulates Temperature
The body’s ability to maintain a consistent internal temperature, known as thermoregulation, is managed primarily by the hypothalamus in the brain. This area monitors the temperature of the blood flowing through it and compares it to a set point, typically around 98.6°F (37°C). When the core temperature deviates, the hypothalamus initiates involuntary responses to generate or dissipate heat.
To warm the body and conserve heat, the hypothalamus triggers shivering, which involves rapid muscle contractions that generate metabolic heat. It also activates vasoconstriction, narrowing blood vessels beneath the skin to reduce blood flow to the surface and minimize heat loss. This action shunts warm blood toward the body’s core organs.
When the body needs to cool down, the hypothalamus activates cooling mechanisms. These include sweating, where moisture evaporation removes heat, and vasodilation, which widens peripheral blood vessels. Vasodilation increases blood flow to the skin, allowing heat to radiate away from the body, often causing the skin to appear flushed.
Fever, Infection, and Immune System Signaling
The most common cause of alternating hot and cold sensations is the body’s response to infection, resulting in a fever. A fever is a defense mechanism where the immune system intentionally raises the body’s temperature to inhibit pathogen growth. This process begins when immune cells, activated by invaders, release fever-inducing chemical messengers called pyrogens.
These pyrogens travel to the hypothalamus, triggering the production of prostaglandin E2, which resets the body’s thermostat to a higher set point (e.g., from 98.6°F to 102°F). Since the actual body temperature is now lower than the new set point, the hypothalamus perceives the body as dangerously cold.
In response, the body initiates intense heat-generating activities like peripheral vasoconstriction and violent shivering, experienced as chills. The person feels cold and shakes uncontrollably as muscles work to raise the core temperature. Once the body temperature reaches the elevated set point, the chills subside, and the person feels hot.
The cycle reverses when the infection clears or medication is taken, resetting the hypothalamic set point to normal. To rapidly drop the temperature, the body engages in intense heat dissipation, leading to drenching sweats as the excess heat is shed.
Hormonal and Metabolic Influences
Temperature fluctuations can arise from imbalances in the endocrine system, which regulates metabolism and heat production. Thyroid disorders are a frequent cause, as thyroid hormones control the basal metabolic rate. Hyperthyroidism (overactive thyroid) accelerates metabolism, leading to excessive heat production and chronic heat intolerance. Hypothyroidism (underactive thyroid) slows metabolism, causing decreased heat production and a persistent feeling of being cold.
Fluctuations of estrogen during menopause and perimenopause commonly trigger hot flashes. The mechanism involves estrogen withdrawal, which narrows the thermoneutral zone in the hypothalamus. This causes the brain to mistakenly believe the body is overheated with only a slight rise in core temperature. This triggers an intense vasodilation and sweating response, creating the sensation of sudden, overwhelming heat.
Low blood sugar (hypoglycemia) is a metabolic event that triggers a hot-and-cold response. When glucose levels drop too low, the body releases stress hormones, primarily adrenaline and norepinephrine, to mobilize stored energy. This surge causes a sympathetic nervous system reaction, resulting in sweating, a feeling of heat, and sometimes a clammy, cold sensation from peripheral vasoconstriction.
Autonomic Nervous System Dysfunction
The autonomic nervous system (ANS) controls involuntary functions like heart rate, breathing, and temperature regulation. Dysfunction in the ANS can cause rapid shifts between hot and cold sensations. The ANS is divided into the sympathetic nervous system (SNS), responsible for “fight or flight,” and the parasympathetic nervous system, which manages “rest and digest.”
Psychological stress, anxiety, and panic attacks cause acute activation of the SNS. This surge of stress hormones initiates a protective response that redirects blood flow to the major muscles. This redirection involves immediate vasoconstriction in the skin, which can make the extremities feel cold and clammy.
This initial cold sensation is often followed by intense heat and sweating as the body’s metabolism speeds up and the hypothalamus attempts to dissipate perceived excess heat. Prolonged stress or anxiety can leave the nervous system dysregulated, stuck in a heightened state of sympathetic activation. This chronic imbalance leads to episodes of rapid temperature changes, such as sudden hot flashes or chills, without infection or fever.