What’s one of the first things that happen when a patient sees a healthcare practitioner? Most likely, the nurse takes the patient’s temperature. Body temperature is one of the four main vital signs that healthcare professionals consistently monitor. It’s “vital” because fluctuations in body temperature can indicate a serious issue. Our very existence relies on the homeostatic process of internal body temperature regulation. (1)

What are body temperature levels?

We’ve all heard someone say, “I run hot” or “I run cold.” And we’ve also had the experience where two people can be in the same room when one feels cold or hot and the other is perfectly comfortable. This is body temperature in action and it’s very individualized.

What regulates body temperature? It’s a complex internal system that balances heat loss with heat production and is dictated by the hypothalamus in the brain. The hypothalamus is like the body’s internal thermostat, making continual adjustments to keep the body within a normal range. A variety of other body factors including the skin, sweat glands, blood vessels, and hormones influence this internal thermostat.

Types of body temperature levels

Body temperature can vary from low to normal to high. Typically, high body temperature begins at about 100 degrees Fahrenheit. If body temperature is consistently below 98.6 degrees Fahrenheit, it could indicate low body temperature and should be evaluated. Before we dig into low and high body temperature, let’s take a closer look at what’s considered normal.

Normal body temperature

In the case of body temperature, “normal” may be a misnomer. In fact, we are now learning that body temperature is highly individualized and can be influenced by many factors including age, gender, circadian rhythm, metabolism, ovulatory cycles, and hormonal balance. “These factors vary dramatically across individuals, raising the possibility that individuals have baseline temperatures that differ systematically from the population average,” wrote the researchers of a 2019 cohort study published in the British Medical Journal. “The same temperature that is normal for one person might be dangerously high for another.” (2)

This type of debate about normal body temperature continues in the medical literature and yet it’s also widely accepted that “normal” body temperature is 98.6 degrees Fahrenheit. When dealing with body temperature, it’s important to address this debate. For example, according to a 2019 systematic review published in the journal Open Forum Infectious Diseases, the normal body temperature range should be 97.9 degrees Fahrenheit to 98.6 degrees Fahrenheit. (3) This is consistent with other reports indicating that normal body temperature should be lower than 98.6.

Jonathan Hausmann, MD, with Boston Children’s Hospital and Beth Israel Deaconess Medical Center led a study to help redefine normal body temperature as it relates to fever. (4) “The assumption is that normal temperature is 98.6 degrees and fever begins at 100.4. But we found the average temperature is more like 97.7, and by defining ‘fever’ as temperature above the 99th percentile of normal, we found fever typically begins at about 99.5 degrees.” (5)

Practitioner insight

“It’s important for practitioners to accept the fact that body temperature is highly individualized,” said integrative health expert Ronald Hoffman, MD. “Just as with many metrics in medicine, we need to get away from a one-size-fits-all approach.”

Clinicians should establish a baseline normal for each of their patients and then monitor each patient accordingly.

High body temperature

When there is a fever, body temperature is elevated. This typically occurs when the body is fighting an infection. However, body temperature can be high during other circumstances such as with exercise, hot and humid weather, eating spicy foods, or consuming caffeine or alcohol. Dehydration and heatstroke are also prime scenarios when the body temperature rises and causes a health crisis.

An overactive thyroid (e.g. hyperthyroidism) can also cause body temperature to rise. In addition to high body temperature, other signs of an overactive thyroid are weight loss, heart palpitations, poor concentration, disturbed sleep, and anxiety. (6) In cases of hyperthyroidism, supplemental selenium (7) and L-carnitine (8) can provide adjuvant support. A low iodine diet and avoiding iodine supplementation is also recommended in cases of an overactive thyroid. (9)

Low body temperature

Conversely, when body temperature is consistently low, it could be a sign of hypothyroidism or low thyroid. Hoffman cautions clinicians, however, that body temperature is just one of several diagnostic factors when it comes to hypothyroidism.

Practitioner insight

“There is this folkloric notion that the Barnes Basil Temperature Test provides a definitive diagnosis of hypothyroidism,” cautions Hoffman. “This has led to a slavish adherence to false ideal body temperature as a determinant of who should get thyroid support and how much, without regard to blood test results and patients’ subjective responses—often putting patients at risk.”

Other symptoms of hypothyroidism can include fatigue, weight gain, cold intolerance, muscle weakness, constipation, depression, and menstrual irregularities. (9) Although iodine supplementation is not recommended in cases of hyperthyroidism, it’s often recommended in cases of low thyroid especially when iodine deficiency is present. (10)

People with low thyroid may also be deficient in vitamin B12, so B12 supplements or injections are worth considering when body temperature is consistently low. (10)

What else causes low body temperature? Of course, hypothermia is when the body temperature drops to dangerously low levels after being out in the cold for an extended period. Sepsis, a very serious and potentially deadly infection, is also an example of when body temperature is too low.

In addition to iodine and vitamin B12 for low thyroid, here’s how to raise body temperature through diet and lifestyle:

• Eat certain thermogenic foods like ginger, cayenne pepper, and green tea to “heat up” the metabolism (thermogenic means to produce energy and in the case of these foods, it also means increased metabolism)
• Stay consistently physically active to encourage blood flow and sweat, which will help bring body temperature up
• Get enough quality sleep because sound sleep will help balance body temperature, as well as many other body systems

The bottom line

It’s time to appreciate fluctuating body temperature and accept the fact that body temperature can be unique to the individual’s physiology, age, gender, and other factors. Once a baseline range is established, monitoring an individual’s body temperature remains vital. Both ends of the low-to-high body temperature spectrum can present serious health issues that must be addressed clinically.