Thyroid, Weight, and BMR
How changes in thyroid function change your body’s metabolism.
by Dr. Vedrana Högqvist Tabor
Thyroid hormones are responsible for how much energy our body can and will spend during the entire day.
Thyroid controls our energy spent in three different ways:
1. By maintaining basal metabolic rate (BMR)
2. By regulating our body temperature
3. By modulating appetite and how much food we eat
Through these, thyroid hormones also regulate our weight.
BASAL METABOLIC RATE (BMR)
BMR is the main way humans spend 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 BMR; the higher TSH, the lower BMR is (2,3). If you are on thyroid medication, your BMR might be extremely sensitive to even small changes in your medication levels (4).
How do thyroid hormones change BMR?
Thyroid hormones increase production of the main energy chemical in the body, called ATP (adenosine triphosphate) (5,6). This process is done in the energy factory of the cell, called mitochondria (7), and it is both the amount of T3 as well as the balance between T3 and T4 that determine how much ATP is made.
What happens when going from an underactive thyroid to balanced thyroid (euthyroid)?
The thyroid hormones in your body, the more heat and ATP your body will produce. In case of medication overtreatment, there will be both more ATP and more heat produced than needed for normal body functions (8). Out of all the thyroid hormones, T3 is the most important for this activity (9).
It has also been shown that treatment with levothyroxine will restore TSH to normal levels, but it does not fully restore lower metabolic rates (10).
Are thyroid hormones the only ones impacting BMR?
Probably not. There are other hormones and other chemicals in the body that are important for maintaining BMR, and they might have a direct impact on metabolism, or on thyroid hormone production.
Through evolution, humans have developed mechanisms to maintain body temperature in order to prevent us from dying in cold environments, but also to increase our energy spent after we have eaten (11). Thyroid hormones are indispensable for this.
A lot of energy and heat is produced by muscles when we exercise, and this ability to work out, as well as to produce energy, is thanks to thyroid hormones. Even when we are walking or just typing an email, thyroid hormones enable our muscles to work (12).
Well-balanced thyroid hormones are key to maintaining a healthy weight, and any changes in hormone levels — even if they are within the reference range — will have an impact on body weight (13, 14). In general, the higher TSH is, the higher the body mass index will (BMI) be. This is the rule of the thumb, but is not always be the case. Also, when on levothyroxine (T4) therapy, while the weight might drop, body fat composition does not change as much (15). A combination of T3 and T4 therapy might be beneficial for weight loss by affecting BMR, as well as changing how hungry we 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 a complex one, as many more hormones and chemicals come into play when regulating metabolism. However, the impact low thyroid production has on the thyroid hormones is very powerful, and a non-functioning thyroid will lead to lowered metabolism, problems with regulating body temperature, and weight gain. All of the three have a longer-term consequences, too. This is why it’s very important to know your BMR, and to know how many calories you burn per day (daily calorie expenditure).
We have added a BMR calculation in the BOOST Thyroid app, so you can get an immediate understanding of how many calories you use when resting. This will help you understand your metabolism and manage your weight.
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