Thyroid, Weight, and BMR
Thyroid hormones are responsible for how much energy your body spends during a day. They control how your energy is spent in three different ways:
1. Maintaining basal metabolic rate (BMR)
2. Regulating your body temperature
3. Modulating appetite and how much food you eat
Thyroid hormones thus affect your weight through these modes of energy regulation.
How do thyroid hormones change basal metabolic rate (BMR)?
BMR is the main way your body spends energy, which means any change in BMR will lead to a change in weight if food intake is not modified accordingly (1). Thyroid hormones are the main regulators of your BMR — the higher your TSH is, the lower your BMR is (2, 3). If you’re on thyroid medication, your BMR might be extremely sensitive to even the smallest changes in your medication levels (4).
Thyroid hormones increase the production of adenosine triphosphate (ATP), the main energy chemical in your body (5, 6). This process is carried out in the mitochondria, which is the main energy factory of the cell (7). Both the amount of T3 as well as the balance between T3 and T4 determine how much ATP is made.
What else impacts BMR?
There are other hormones and chemicals in your body that are important for maintaining BMR. It’s likely that they have a direct impact on metabolism or on thyroid hormone production.
Going from an underactive thyroid to balanced thyroid (euthyroid)
The more thyroid hormones there are in your body, the more heat and ATP your body will produce. Medication overtreatment contributes to overproduction of ATP and heat, making more than what’s needed for normal bodily functions (8). Out of all thyroid hormones, T3 is the most important for this activity (9).
Treatment with levothyroxine will restore TSH to normal levels, but it doesn’t fully restore lower metabolic rates (10).
Body temperature and thyroid hormones
Through evolution and thyroid hormones, people have developed mechanisms to maintain body temperature in order to survive in cold environments — as well as to increase energy spent after eating (11).
A lot of energy and heat is produced by muscles while exercising. Whether walking or just typing an email, thyroid hormones enable your muscles to work (12).
Body weight and thyroid hormones
Well-balanced thyroid hormones are key to maintaining a healthy weight. Any changes in thyroid hormone levels — even if they’re within a normal reference range — will have an impact on your body weight (13, 14).
Generally high TSH levels correlate with higher body mass index (BMI). People on levothyroxine (T4) therapy may find that while they might lose weight, their body fat composition doesn’t change as much (15). A combination of T3 and T4 therapy can be beneficial for weight loss — by affecting your BMR and changing how hungry you feel (16). Other hormones such as the hunger hormone leptin might be involved in this process too (17).
The relationship between thyroid hormones, metabolism, and body weight is complex. Lots of other hormones and chemicals can play a role in regulating your metabolism. However, the impact of low thyroid hormone production is very powerful. A non-functioning thyroid will lead to lowered metabolism, problems with regulating body temperature, and weight gain — these side effects have long-term consequences too.
That’s why it’s important to know your BMR and how many calories you burn per day (daily calorie expenditure).
How we write: our information is based on the results of peer reviewed studies using the National Library of Medicine platform. It is written by scientists and reviewed by external experts. If you believe we might have overseen crucial scientific information, please contact us at firstname.lastname@example.org
Disclaimer: This information is not intended to mitigate, prevent, treat, cure or diagnose any disease or condition. If you want to change your treatment, lifestyle, your diet, include supplements in your diet or have concerns about your health, please consult your doctor before trying new approaches.
- Ravussin E, et al. Reduced rate of energy expenditure as a risk factor for body-weight gain, 1988
- Danforth E, et al. The role of thyroid hormones in the control of energy expenditure, 1984
- Silva JE. The thermogenic effect of thyroid hormone and its clinical implications, 2003
- Al-Adsani H, et al. Resting energy expenditure is sensitive to small dose changes in patients on chronic thyroid hormone replacement, 1997
- Freake HC, et al. The regulation of lipogenesis by thyroid hormone and its contribution to thermogenesis, 1989
- Silva JE. Thermogenic mechanisms and their hormonal regulation, 2006
- Hafner RP, et al. Altered relationship between protonmotive force and respiration rate in non-phosphorylating liver mitochondria isolated from rats of different thyroid hormone status, 1988
- Harper ME, et al. The quantitative contributions of mitochondrial proton leak and ATP turnover reactions to the changed respiration rates of hepatocytes from rats of different thyroid status, 1993
- Flandin P, et al. Uncoupling protein-3 as a molecular determinant of the action of 3,5,3′-triiodothyronine on energy metabolism, 2009
- Gorman C.A. et al. Comparative effectiveness of dextrothyroxine and levothyroxine in correcting hypothyroidism and lowering blood lipid levels in hypothyroid patients, 1979
- Cannon B, et al. Brown adipose tissue: function and physiological significance, 2004
- Simonides WS, et al. Thyroid hormone as a determinant of metabolic and contractile phenotype of skeletal muscle, 2008
- Fox CS, et al. Relations of thyroid function to body weight: cross-sectional and longitudinal observations in a community-based sample, 2008
- Knudsen N, et al. Small differences in thyroid function may be important for body mass index and the occurrence of obesity in the population, 2005
- Karmisholt J, et al. Weight loss after therapy of hypothyroidism is mainly caused by excretion of excess body water associated with myxoedema, 2011
- Celi FS, et al. Metabolic effects of liothyronine therapy in hypothyroidism: a randomized, double-blind, crossover trial of liothyronine versus levothyroxine, 2011
- Blum WF, et al. Human and clinical perspectives on leptin, 1998
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