Basal body temperature and thyroid function
Why your temperature might be lower than average
Having an underactive thyroid changes the way body heat is generated and maintained. This is why most people with an underactive thyroid and Hashimoto’s will have lower body temperatures than average.
Keeping up basal body temperature (BBT) is one of the most costly processes in the body. Many hormones in the body may impact it, but thyroid hormones are the only ones that modulate BBT to correspond to the energy balance in the body. Since an underactive thyroid means a lack of energy, that is one of the reasons the BBT of people with an underactive thyroid will be lower (1).
What is BBT?
Basal body temperature (BBT) is the temperature of your body at rest. It is usually the lowest body temperature you will experience throughout the day.
Thyroid hormones are pivotal for BBT generation and maintenance, and even slight changes in hormone levels will show the change in BBT (1).
How does the thyroid regulate BBT?
Thyroid hormones regulate body temperature by increasing available energy in the body, as well as by increasing appetite, pulse, the amount of oxygen delivered to different body parts, and fat buildup. Thyroid hormones work together with the nervous system in order to maintain body temperature.
Thyroid hormones determine how much of the energy molecule called ATP (adenosine triphosphate) will be produced by the body (2). ATP is the energy currency in the cell, and the more you have it, the better you will be at generating heat. If you generate too much heat, you start sweating; if too little, the muscles will start to shiver.
The difference in the amount of energy made by the cell might be quite striking: for any type of muscle work, for example walking or running, people with an underactive thyroid might generate up to three times less heat, when comparing to people with a normally functioning thyroid (3, 4).
This can be corrected with thyroid medication, and it has been shown that even minimal changes in fT4 levels (within normal ranges) can have a significant impact on the body temperature (1, 5, 6).
Where is heat created?
Most basal heat is made in brown fat, or brown adipose tissue. It is important for adapting to cold weather, as are thyroid hormones (1). Brown fat contains a lot of enzymes called deiodinases, which are important for the conversion of T4 into the active T3. This means that brown fat can generate T3, which is crucial for producing the energy molecule ATP, and for creating heat during that process (7).
The thyroid and other body parts
Thyroid hormones might increase the heat-utilizing hormonal impact on the nervous system (8). They activate adrenaline and noradrenaline and through this increase T4 to T3 conversion (6). Most notably, when starving, T4 to T3 conversion drops (9).
Increased temperature also increases the need for more oxygen, which increases heart function in order to get more oxygen to different parts of the body (10).
Other causes of changes in BBT
Thyroid hormones are the main regulators of body temperature, but not the only ones have an impact: insulin, glucagon, estrogen and progesterone, epinephrine, leptin and ghrelin have a major impact on BBT (1). While the thyroid can have a more continuous impact on the BBT, the other hormones are more temporary.
Genes have a big impact on how a person’s BBT is regulated (11), and they pre-determine the BBT level.
How does cold exposure impact BBT?
During exposure to cold, thyroid hormones activate brown fat (12). With an underactive thyroid, the body develops protective mechanisms to preserve heat, and one might feel this as cold hands and feet. This mechanism is also governed by thyroid hormones (1, 13).
Why track BBT?
Basal body temperature is a good overall indicator of the thyroid status, and might tell more about how well medications are compensating for thyroid activity. It can be a first indicator of a medication overdose.
You can track your BBT in the BOOST Thyroid app and have a better overview of how your thyroid function impacts your body.
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Kahala GJ, et al. Cardiovascular hemodynamics and exercise tolerance in thyroid disease, 2002
R. Rising, et al. Concomitant interindividual variation in body temperature and metabolic rate, 1992
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Castillo M, et al. Disruption of thyroid hormone activation in type 2 deiodinase knockout mice causes obesity with glucose intolerance and liver steatosis only at thermoneutrality, 2011