Does Taking Levothyroxine Help With Weight Loss?

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Just about anyone who has wanted to lose weight and has found it difficult—and let’s face it, that includes most of us—has wondered whether taking thyroid hormone, such as levothyroxine, might help things along a bit. After all (we might reason), everyone knows that people with an underactive thyroid gland usually gain weight, and people whose thyroid glands are overactive are said to lose weight.

Obviously, then, thyroid hormone levels play an important part in determining a person’s weight.

So, wouldn’t it make sense to ask our doctors to prescribe just a bit of extra thyroid hormone, just enough to goose our weight loss a little, and get us started in the right direction?

It turns out we are not the first ones to ask this question. The use of “extra” thyroid hormone as a means of producing weight loss was considered (and, for a time, was used) in the past. There are two reasons it is generally not used today.

First, studies have shown that it does not work. Second, experience shows that there are significant risks to taking “extra” thyroid hormone.

What Is the Relationship Between Thyroid Hormone and Weight?

The level of thyroid hormone in the body is a critical factor in regulating our overall metabolism.

Our metabolism is essentially a measure of how much oxygen we burn in living our daily lives, and how much energy we consume in doing so.

The higher our metabolism, the more energy we are using; that is, the more calories we are burning.

Whether we gain or lose weight is, fundamentally, a matter of caloric balance. Caloric balance is determined by how many calories we absorb into our bloodstream (i.e., what we eat), minus how many calories we burn (i.e., our overall metabolism).

So, one way to lose weight would be to increase the number of calories we burn. And the most straightforward way to use up more calories would be to increase our daily activity levels.

Another way to do it, if it could be done safely, would be to increase our basal metabolic rate (BMR)—essentially the number of calories we burn while we are at rest. This is where, in theory, thyroid hormone plays a role in determining a person’s weight.

To a great extent, our BMR is a function of the level of thyroid hormone in our bloodstream. In fact, in earlier times (before blood tests were available to measure thyroid function), measuring BMR was a useful way to assess a person’s overall thyroid function. Low BMRs were associated with underactive thyroid function, and high BMRs were associated with overactive thyroid function.

And sure enough, many people who develop hypothyroidism find themselves gaining weight, while many who have hyperthyroidism will lose weight.

So it all seems pretty simple, doesn’t it? Anyone who wants to lose weight should just take a little extra thyroid hormone, increase their caloric expenditures to push their caloric balance into the negative range, and the weight should start to come off—no?

And this is exactly the kind or reasoning doctors used at one time to prescribe thyroid hormones for weight loss. Unfortunately, the results when they did so were usually quite disappointing.

Why Taking Extra Thyroid Is Less Effective Than You Might Think

People with normal thyroid function who have taken thyroid hormones in an attempt to lose weight have typically not lost much, if any, substantial weight. There are at least two reasons this has been the case.

First, while thyroid hormone levels are an important determinant of metabolism, they are not the only determinant. Weight gain or loss is actually determined by the complex interplay among numerous physiologic factors, of which the thyroid hormones are just one.

These several physiologic factors act on our gastrointestinal tracts, other hormone systems, and various parts of our brain to modulate both our energy expenditures and our caloric intake.

It is very difficult to predict what will happen when we change one single aspect of this complex system, such as our thyroid hormone levels. Indeed, it is impossible to say what will happen to a specific person’s weight when you give them thyroid hormone. Most typically, it turns out, not much happens.

Second, studies have now suggested that giving levothyroxine (T4) even in high doses—high enough to completely suppress TSH levels, which is done in many people who have been treated for thyroid cancer—does not result in an increased BMR at all, as compared to “normal” controls. In other words, pushing thyroid hormones hard enough to drive TSH even to very, very low levels cannot be relied on as a way of substantially increasing the BMR. It is possible that administering T3 in addition to T4 might give a different result, but most doctors are reluctant to use anything except T4 in treating thyroid conditions.

Finally, we should look at the real-world experience of people who have been diagnosed with hypothyroidism, and are subsequently treated with thyroid hormones. Most of these people have become overweight, and they (and their doctors) imagine that their excess weight will simply melt away once their thyroid hormones have been adequately replaced. And sometimes that actually happens. But far more often, studies have shown, these individuals fail to lose much weight, if any, and all too often they will even gain more weight as their thyroid hormone levels are normalized.

Why does that happen? Part of the answer may be that treatment with T3 in addition to T4 is necessary in some people to increase the BMR, so treatment with T4 alone may not be sufficient. But even in hypothyroid people who are treated with T3, and whose TSH levels have been pushed down to the lower part of the normal range (indicating adequate thyroid replacement), substantial weight loss often proves to be extremely difficult.

What’s happening, most likely, is that when you replace thyroid hormones in overweight people with hypothyroidism, you are increasing their BMR to some extent, but not enough to make them lose substantial amounts of weight. When all is said and done, you have merely converted them from being overweight hypothyroid people to being slightly less overweight euthyroid (that is, normal thyroid) people. They have been converted to your typical, non-hypothyroid, overweight person, a person who weighs too much because of poor diet, reduced activity levels, and/or genetic factors. So, the treated hypothyroid individual finds herself in the same position as an overweight person with normal thyroid function. And she finds it is just as difficult to lose the weight.

This very, very common scenario should tell us loud and clear that, while thyroid hormone is important to our metabolism, it is not a panacea for weight loss.

Why Taking Extra Thyroid Causes Problems

Aside from the fact that taking extra thyroid hormone is not very effective at producing significant weight loss, there are also risks to doing so. Among these are cardiac arrhythmias (including atrial fibrillation), a loss of bone density, a reduction in skeletal muscle mass, and anxiety disorders. Normal amounts of thyroid hormones are necessary for our health, but “extra” thyroid hormones can cause serious problems.

A Word From Verywell

For people with hypothyroidism, replacing thyroid hormones is necessary to restore health, but is often not very effective in producing desired weight loss. If you are not hypothyroid, taking thyroid hormone in an effort to lose weight is not only very likely to fail, but also would expose you to significant adverse effects.

Sources:

Hoogwerf BJ, Nuttall FQ. Long-term Weight Reduction in Treated Hyperthyroid and Hypothyroid Subjects. Am J Med. 1984 Jun;76(6):963-70.

Jensen MD, Ryan DH, Apovian CM, et al. 2013 AHA/ACC/TOS Guideline for the Management of Overweight and Obesity in Adults: a Report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines and The Obesity Society. Circulation 2014; 129:S102.

Samuels MH, Kolobova I, Smeraglio A, et al. Effects of Levothyroxine Replacement or Suppressive Therapy on Energy Expenditure and Body Composition.Thyroid. 2016 Mar 1; 26(3): 347–355. doi: 10.1089/thy.2015.0345

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