Factors affecting body temperature

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Factors Affecting Body Temperature

Normal adult core temperature is approximately 37°C (98.6°F), though values of 36-37.5°C are normal in healthy, active individuals. The body maintains this narrow range through a thermoregulatory network centered in the hypothalamus, balancing heat production against heat loss via radiation, conduction, convection, and evaporation.

1. Circadian Rhythm (Time of Day)

Body temperature follows a regular circadian cycle with a variation of ~0.5-1°C. It is lowest between 3:00-6:00 AM and peaks between 3:00-6:00 PM. This rhythm is governed by neurons in the suprachiasmatic nucleus of the anterior hypothalamus, entrained by light-dark cues. It operates independently of the sleep-wake cycle.

Medical Physiology (Boron & Boulpaep), p. 1537
Ganong's Review of Medical Physiology, p. 322

2. Age

Newborns: Thermoregulation is immature. Infants cannot shiver or sweat effectively, and their high surface-area-to-mass ratio makes them highly susceptible to environmental temperature swings. They compensate via brown adipose tissue (BAT) stimulated by the sympathetic nervous system.
Older adults: Show greater temperature fluctuations due to progressive loss of thermal sensation, reduced muscle mass (less heat generation), lower metabolic rate, reduced cardiovascular reserve, and sweat gland atrophy.
Children may normally run ~0.5°C above the adult norm.
Medical Physiology, p. 1541-1563
Ganong's Review of Medical Physiology, p. 322

3. Sex and Menstrual Cycle

In women, there is an additional monthly cycle overlaid on the circadian rhythm. An abrupt rise of 0.3-0.5°C occurs at ovulation and is sustained through the postovulatory (luteal) phase. This is driven by progesterone acting on the CNS thermoregulatory network and can be used as a fertility guide.

Medical Physiology, p. 1539
Ganong's Review of Medical Physiology, p. 322

4. Exercise and Physical Activity

Muscular work is the single greatest variable source of heat. The body's heat production can rise from ~70 kcal/hr at rest to ~600 kcal/hr during vigorous exercise. This substantially elevates core temperature during activity. Even at rest, increased voluntary movement (e.g., foot-stamping in cold weather) generates measurable heat.

Medical Physiology, p. 1566
Ganong's Review of Medical Physiology, p. 321

5. Basal Metabolic Rate (BMR) and Thyroid Hormones

All vital metabolic processes continuously produce heat. Any factor that raises BMR raises temperature:

Hyperthyroidism: Excess T3/T4 elevates BMR, increases ATP turnover, O2 consumption, and body temperature.
Hypothyroidism: Reduced BMR lowers heat production and body temperature.
Catecholamines (epinephrine, norepinephrine) released in cold or stress also increase thermogenesis.
StatPearls - Physiology, Temperature Regulation
Ganong's Review of Medical Physiology, p. 322-324

6. Environmental (Ambient) Temperature

External temperature directly influences heat exchange between the body and its surroundings through:

Radiation - largest pathway (~50-60% of heat loss at rest)
Conduction/Convection - wind speed greatly enhances heat loss
Evaporation - becomes dominant as ambient temperature approaches body temperature

As environmental temperature rises toward body temperature, radiation losses decline and sweating (evaporative loss) takes over as the primary cooling mechanism.

Ganong's Review of Medical Physiology, p. 321-322

7. Stress (Physical and Emotional)

Both physical and psychological stress elevate temperature via hormonal and neurological stimuli that increase metabolism. Increased secretion of norepinephrine and epinephrine raises thermogenesis. The well-known slight temperature rise seen in hospitalized patients on admission day reflects excitement and apprehension.

Ganong's Review of Medical Physiology, p. 322

8. Infection and Fever (Pyrogens)

Fever is an elevation in the hypothalamic set-point (not a failure of regulation). Exogenous pyrogens (bacteria, viruses) stimulate release of endogenous pyrogens (e.g., IL-1, IL-6, TNF), which trigger prostaglandin E2 synthesis in the hypothalamus, raising the set-point. The body then generates heat (shivering, vasoconstriction) to reach the new set-point.

9. Measurement Site

The site of measurement significantly affects recorded temperature:

Site	Typical Value	Notes
Rectal	~37°C (reference)	Most accurate core estimate
Oral	~0.5°C lower than rectal	Affected by hot/cold drinks, chewing, smoking, mouth breathing
Axillary	~1°C lower than rectal	Least accurate
Tympanic	Approximates core	Convenient but technique-dependent

Ganong's Review of Medical Physiology, p. 322

10. Hormonal Factors (Beyond Thyroid)

Estrogens/Progesterone: Govern the menstrual cycle fluctuation (see above)
Epinephrine/Norepinephrine: Trigger vasoconstriction and BAT thermogenesis in cold
TSH: Cold increases TSH secretion in animals; significance in adult humans is modest

11. Clothing and Behavior

Behavioral thermoregulation is highly effective: wearing warm clothing, curling up (reducing exposed surface area), seeking shade or shelter, and adjusting food intake (hunger increases in cold - a semiconscious drive to fuel thermogenesis) all modulate temperature.

Summary Table

Factor	Effect on Temperature
Circadian (6 PM)	Peak (+0.5-1°C)
Circadian (3-6 AM)	Trough
Exercise	Raises significantly
Ovulation/Luteal phase	Raises +0.3-0.5°C
Fever/Infection	Raises (set-point shift)
Hyperthyroidism	Raises
Hypothyroidism	Lowers
Hot environment	Raises
Cold environment	Lowers
Newborn/Elderly age	More variable
Stress (emotional/physical)	Raises

Sources: Ganong's Review of Medical Physiology, 26th ed. - Physiology, Temperature Regulation; Medical Physiology (Boron & Boulpaep)

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