Why is it so hard to lose weight with Hashimoto’s?

Slow metabolism makes for harder work(outs).

by Dr. Vedrana Högqvist Tabor

Having an underactive thyroid condition makes it more challenging to control or lose weight. I tried with limited success for years, even before I knew I had Hashimoto’s.

Shedding weight comes down to the math: energy in (eaten) vs energy spent (out). But calculating how much energy I have actually used up is not easy, nor it is a constant number. It changes as I age and as my thyroid health and health in general change.

I learned that the first thing to know to successfully lose weight is my own basal metabolic rate (BMR). Knowing how much energy I am using up is a necessary start to make the right lifestyle adjustments and be successful long term.

What is basal metabolic rate?

Basal metabolic rate (BMR) is the rate of energy one’s body spends during a minute, hour or a day to maintain basic body functions, such as breathing, pumping blood, maintaining body temperature and brain function. It is the metabolic rate of our bodies during resting periods, and it accounts for up to ¾ of all energy spent throughout the day.

BMR is usually expressed in calories (kcal), and determines for each one of us if we will maintain, gain or lose weight.

There are average charts for people of a certain height, weight, age and biological sex. These charts are based on averagely healthy population, and can be about 20% off for people with Hashimoto’s.

What determines basal metabolic rate?

In healthy individuals, basal metabolic rate is impacted by:

Body size and composition: If you are taller or heavier, your BMR will be higher. If you have more muscle than fat mass, your BMR will be higher.

Biological sex: On average females have 10% lower BMR compared to males.

Age: After the age of 30, increasing age impacts BMR.

Where you live: If you live in tropical or cold climate, your body will spend extra energy to keep your body temperature at its level. That will increase your BMR.

Thyroid hormone levels: Too low T4 and T3, as well as the wrong ratio, will slow down BMR.

Inflammation: Inflammation from fever or injury will increase BMR.

Thyroid and BMR

Measuring basal metabolic rate used to be one of the earliest tests doctors performed to assess a patient’s thyroid functionality.

This test is no longer used, as BMR can change on a day-to-day or even hour-to-hour basis and is impacted by many processes in our bodies that might be independent of our thyroid status. Basically, one measurement of BMR might not tell us much, but a series of daily measurements throughout a month might give us a good average value, which is close enough to our true BMR value.

Thyroid hormones impact our basal metabolic rate. Low levels of thyroid hormones will decrease our BMR, while the excess of thyroid hormones, especially T3, will increase the BMR (1–4). It is not only the amount of T3 or T4, but also their ratio (T3:T4) that is important for regulating metabolism rates (5).

So, how does our body regulate BMR?

It does so through deiodinases, enzymes whose function is to convert T4 to T3. Deiodinases become active after we intake food and the process of digestion starts. This is when our body signals that we need to have enough T3 made from T4 (6). A similar process happens after the exercise in our muscles: deiodinases help convert T4 into T3, so that T3 will help with muscle growth and maintenance (7, 8). For this T4 in T3 conversion to happen, our bodies need minerals such as selenium, which has been shown to be important for proper metabolism(9).

Thyroid hormones activate energy-producing processes in mitochondria (10, 11). Mitochondria are organelles within the cell, and are known as the powerhouse of the cell. They break down nutrients and create energy for the cell in the form of adenosine triphosphate (ATP). ATP is a molecule with which energy is transferred within the cell. If there is not enough thyroid hormones, not enough ATP will be produced and cells metabolism will slow down.

Thyroid replacement therapy

Although levothyroxine restores normal T4 levels, it does not restore BMR to it’s normal levels. Metabolism remains 10- 20% less efficient, which may cause problems with maintaining a healthy weight (12, 13).

Five no-go(s) when losing weight with an underactive thyroid

  1. Starving yourself — By eating an extremely low calorie diet, you are pushing your body into starvation mode. As a result, the body will try to conserve as much energy as possible, because it does not know when will it have enough food again. By entering starvation mode your body actually needs less calories than in a non-starvation mode.

  2. Counting every calorie — you might want to do it, but it could be counterproductive. Rather try to understand what foods are good for you and how many calories each meal has. Eating a meal that has 30 calories less instead of eating a meal that has a good nutrient composition might work against you in the long run.

  3. Weighing yourself every day — the number on the scale will not always go down, and this might discourage you. Weight on a day-to-day basis can change for other than diet reasons and it does not reflect the work you have put in losing weight. Instead, try weighing yourself once a week. Weekly measurements should be a better reflection of your progress.

  4. Overexercise — you run the risk of using up all the available fT3 in your body. As a consequence,there is not enough T3 to run your metabolism and use up the food you have eaten. You definitely need to work out, be active and move, but do not overdo it. If you make medium-intensity exercise your routine, and lead an active life, your BMR will increase and you will use up more calories throughout the day.

  5. Drinking alcohol — alcohol directly blocks thyroid function, blocks both fT4 and fT3, and increases rT3 (12). This will lead to lowering your BMR.

If you have lost 10% or more of your weight, visit your healthcare practitioner to see if it is necessary to re-adjust your thyroid medication. The risk of losing weight and staying on the same thyroid medication dose is to overdose on thyroid medication. If you start experiencing some of the symptoms of hyperthyroidism, including excessive sweating, rapid or irregular heart beats, and/or short sleeping cycles, contact your health care practitioner as soon as you can.

How to calculate your BMR?

I looked into my average food intake through years in my food diaries. I realized I was eating on average 250 kcal per day more than my BMR allowed me during the times I believed I was eating a balanced diet. To give you a feel of what this number means: 250 kcal/day turns into 1 kg or 2.2 lbs of extra weight in a month.

Calculating BMR is complicated. You will need to take in consideration your age, biological sex, fat-free mass, and your thyroid hormone levels. As you age, your BMR will decrease on average between 1% and 2% per 10 years of your life, starting in your 30s. Depending on your thyroid functionality, your BMR will change too.

If you use BOOST Thyroid App your BMR will be calculated automatically on a daily basis.


  1. Nicol SC et al. Seasonal variations in thyroid hormone levels in free-living echidnas. 2000

  2. Silvestri E et al. Thyroid hormones as molecular determinants of thermogenesis. 2005

  3. Johnstone AM et al. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. 2005

  4. Kim BT. Thyroid hormone as a determinant of energy expenditure and the basal metabolic rate. 2008

  5. Mortoglou A et al. The serum triiodothyronine to thyroxine (T3/T4) ratio in various thyroid disorders and after Levothyroxine replacement therapy. 2004

  6. Kim B. Thyroid hormone as a determinant of energy expenditure and the basal metabolic rate. 2008

  7. Simonides WS et al. Thyroid hormone as a determinant of metabolic and contractile phenotype of skeletal muscle. 2008

  8. Gereben B et al. Cellular and molecular basis of deiodinase-regulated thyroid hormone signaling. 2008

  9. Arthur JR et al. Selenium deficiency, thyroid hormone metabolism, and thyroid hormone deiodinases. 1993

  10. Harper ME et al. Thyroid hormone effects on mitochondrial energetics. 2008

  11. Lanni A et al. Mitochondria actions of thyroid hormone. 2016

  12. Cotton GE et al. Suppression of thyrotropin (h-TSH) in serums of patients with myxedema of varying etiology treated with thyroid hormones. 1971

  13. Gorman CA et al. Comparative effectiveness of dextrothyroxine and levothyroxine in correcting hypothyroidism and lowering blood lipid levels in hypothyroid patients. 1979

  14. Balhara YPS et al. Impact of alcohol use on thyroid function. 2013