This summary includes research published in peer-reviewed journals including Frontiers in Bioscience, Archives of Internal Medicine, Physiological Reviews, and more.
We humans differ from animals and birds in a unique way: we can maintain a constant body temperature and don’t require hibernation or migration to warmer climates.
As simple as this may sound, maintaining body temperature has taken millions of years to evolve.
Beyond a certain amount of heat generated from metabolism, this requires a system that can generate additional heat in order to raise body temperature (e.g., to 37° C for humans, 42° C for birds).
During cold, as outside temperature drops, blood vessels shrink to reduce heat loss and people curl up like a sphere, to minimize the exposed area of the body (sphere has minimum surface area for a given size).
In parallel, the body starts generating heat. At first through shivering, but then the baseline metabolic rate increases. Over time, the brown fat deposited around vital organs such as kidney, spinal cord and blood vessels starts to generate enough heat to raise the body temperature.
What if the outside temperature is too high?
We have developed unique processes to dissipate heat and lower the body temperature. Sweating to cool down, and slowing down of certain biological processes to reduce heat generated in the body are just few examples. (That’s one reason why people eat spicy food in hot climates, because it increases sweating, which cools the body.)
The baseline metabolic rate, or BMR, is the lowest amount of energy necessary to stay alive. This would ideally be the minimum energy our body needs.
It is measured at rest, after a meal, and at ambient temperature when no work is needed to heat or cool the body in response to outside temperature fluctuations.
Since we often live indoor at 20-25° C, our body is constantly burning stored fat to keep the temperature at 37° C.
In animals that live in cold environments, more body heat is necessary to maintain the temperature. It comes from a higher baseline metabolic rate and constant heat generated from the stored fat.
The baseline metabolic rate—sometimes also called, resting energy expenditure—depends on size, genetics, and many other factors including age, pregnancy, and gender.
Thyroid is one of the main knobs to maintain a tightly controlled temperature around a 37° C (or 98.6° F). Even birds and other mammals use thyroid hormones to balance their body temperature.
During cold, thyroid hormones, T4 and T3 will stimulate the body to generate more heat.
In hot weather, when outside temperature rises above our body temperature, the release of TSH slows down. T4 and T3 already circulating in the blood lose an iodine atom and convert into non-active forms, e.g., T4 into reverse T3 (rT3).
What happens if the system malfunctions and the body continues to produce thyroid hormones?
Hyperthyroidism is such a condition when excess thyroid hormone, T4, circulates in the blood. As a consequence, the body constantly struggles to lower the temperature. This results in common symptoms of fatigue, high sensitivity of heat, irritation, and weight loss.
On the other hand, in hypothyroidism, our body can not supply enough thyroid hormones to maintain the temperature. A typical symptom is continuous feeling of cold.
Initially, TSH levels rise (or drop) to maintain sufficient T4 and T3 levels. However, beyond a certain level when TSH levels saturate (or bottom out), the system malfunctions, resulting in a thyroid disorder.
The so called brown adipose tissue—the brown fat distributed around key organs such as liver, heart, kidneys, etc.—is one of the key players in maintaining body temperature. On exposure to cold, these BAT cells generate the necessary heat to raise the temperature.
In hypermetabolic state—in hyperthyroidism—resting energy spending increases, people lose weight, their cholesterol levels drop, brown and white fat burning increases, and a higher blood sugar appears. These processes reverse in case of hypothyroidism.
Technical info – how a signal of feeling cold, translates into heat generation in the body?
As soon as the skin senses cold, blood vessels shrink, and the sympathetic nerves send signal to hypothalamus. This causes shivering, and the brown fat surrounding blood vessels and key organs receive signal to activate their adrenaline receptors (norepinephrine). This results in fat burning (lipolysis) to release heat in the body. Thyroid hormones and the uncoupling protein (UCP1) rapidly activate by lipolysis and the cell mitochondria oxidation results in heat generation.
Have you noticed the feeling of slump after a meal?
That’s because carbohydrate metabolism and the resulting insulin acts as a switch to activate the enzyme responsible for body heat generation.
Fasting slows down the supply of thyroid hormones to avoid any fat burning in the body—a behavior similar to hypothyroidism.
In diabetes, the insulin resistance affects body’s ability to stay warm through heat generation from the process of burning brown fat tissues.
In animals that have thyroid dysfunction—and can not properly control the process of fat burning to generate heat—continuous eating is essential to keep them warm.
During extreme starvation, the body shuts down this heat producing mechanism. Similar slow down occurs in hibernation, which is also mediated by thyroid hormones.
Thyroid hormones play a key role in controlling metabolism together with brain, white fat, brown fat, skeleton muscles, liver, and pancreas. That’s why they are considered potential paths to solve the metabolic disorders related to obesity, diabetes, and high cholesterol. A thyroid test is one of the first step to understand the underlying issues.
Low thyroid hormone levels are also associated with retaining water in the body. On treatment, release of this excess water results in weight loss (amount of fat generally remains the same).
Studies suggest hyperthyroidism increases craving for carbohydrates which returns to normal after treatment of high thyroid hormones levels.
Interestingly, T3 is about ten-times more active than T4 in the body. That’s why it is seems to be more effective in weight loss and lowering cholesterol. However, no effect on insulin or cardiovascular health occurs.
A tiny gland in the throat has developed as a vital organ to maintain such a complex system of temperature control is truly an amazing feat of evolution.
Thyroid Hormone Regulation of Metabolism by Mullur et. al., Physiological Reviews, Vol 92, Apr 2014.
Physiological importance and control of non-shivering facultative thermogenesis by J. Enrique Silva, Frontiers in Bioscience, S3, Jan 2011.
Relations of Thyroid Function to Body Weight: Cross-Sectional and Longitudinal Observations in a Community-Based Sample by Fox et. al., Archives of Internal Medicine, Vol 168, Mar 2008.
Mechanisms of thyroid hormone action by Gregory A Brent, The Journal of Clinical Investigation, Vol 122, Sep 2012.