The Thyroid Gland Part 1 – Fat Loss, Muscle Growth, and Diet

In this series I am going to write about the thyroid gland and the hormones that it produces (or that it no longer produces). Although the term “thyroid”‘ is often associated with fat loss, the thyroid also helps to gain or lose muscle. It therefore has other functions which will be described after we have established some simple ideas of physiology. I also reserve a large place for explaining why it slows down during a diet and how to restart it in a natural way.

The thyroid, how does it work?

The thyroid gland is situated in the top part of the neck. It produces several hormones which are crucial for the appearance of the bodybuilder. It is not a question here of flaunting our science with complicated terms. I am concentrating on what is important for the bodybuilder in terms of leanness and muscles, but it is important to understand how the thyroid works and its mechanisms of action.

The thyroid gland produces a substance called T4 (thyroxine or tetra-iodothyronine). This T4 is called a pro-hormone, as the scientists haven’t succeeded in demonstrating a direct action by it. Of course, a pro-hormone, is nonetheless a hormone. But its action is indirect, as it will, in part, be transformed into T3 (triiodothyronine) which is responsible for the actions of the thyroid. Not all the T4 is transformed into T3 — it therefore constitutes a reserve of thyroid hormones in the blood. Its lifetime will thus be longer, the T3 being degraded more rapidly.

A portion of the T3 (20%) comes directly from the thyroid, the rest coming from the transformation of the T4. Tissues like the liver, the kidneys, the heart, the muscles or the central nervous system are the sites of this transformation.

The action of TSH

TSH is a hormone that stimulates the thyroid (Thyroid Stimulating Hormone) to produce T4 and T3. When these two are elevated, they lower the level of TSH. On the other hand, when T3 and T4 are low, the level of TSH increases. In a blood analysis, a hypothyroid is therefore in theory characterized by a low T3 and T4 and a raised TSH, while a hyperthyroid is characterized by an elevated T3 and a low TSH.

In reality, many hypothyroids have not only a low TSH, but also a low T3. This is called having a “lazy” thyroid as the body does not react to the deficiency of thyroid hormones. Women often have this condition.

It is said that TSH determines the size of the thyroid gland. Those who take medications containing thyroid extracts lower the level of TSH, which means that they will atrophy their thyroid gland, sometimes even for a long time after the stopping of this medication. But why is this gland so important for the bodybuilder?

What are the effects of thyroid hormones?

First of all, they are a determining factor for basal metabolism. The more the T3 is elevated, the more calories the body burns, particularly in the form of heat. Therefore the higher the level of these hormones is elevated, the more your body temperature will be elevated. In consequence, taking your temperature in the morning is a good indicator of the level of activity (or lack of activity) of your thyroid. Many women find that they are constantly cold, even in warm weather. This is often because they suffer hypothyroidism.

The thyroid hormones are also at once anabolic and catabolic. A weak level of T3 is mainly anabolic and very little catabolic. The higher this level gets, the more the catabolic action will predominate, while the anabolic activity reaches a ceiling quickly. The thyroid hormones also have indirect actions. For example: only the action of either epinephrine or norepinephrine on the beta receptors in the fat cells has lipolytic (fat loss) properties. So if one blocks these beta receptors, the increased caloric expenditure due to a high T3 level will be derived mostly from muscles (amino acids), and not from fat. On the other hand, thyroid hormones help to reduce fat (and to gain muscle) by increasing the number of beta receptors in the fat and muscle cells.

Moreover, thyroid hormones are indispensible for the production of IGF-1 by the liver. A large part of this permissive effect, other than a stimulation of the secretion of the secretion of GH, is due to the increase of the number of growth hormone receptors in the liver. Of course, all this works well if the thyroid gland functions normally. Unfortunately, this isn’t always the case, especially during a diet poor in calories. We must therefore ask:

What are the effects of a diet on the thyroid gland?

This is not a secret for anyone: the stricter and longer a diet is, the more the body adapts to the deficiency in calories and reduces its energy needs. Metabolism slows down and the body temperature is lowered. This action is essentially due to a reduction in the level of thyroid hormones in the blood. The lowering of thyroid hormones is produced not only directly by an atrophy of the gland but also by a lessening of the transformation of T4 to T3.

How is this lowering of activity of the thyroid gland produced?

  • Many hormones have a permissive effect on the action of TSH. The best known of these hormones are insulin and IGF-1. When one eats less, the plasma levels of these two hormones drop. The result is that TSH is unable to stimulate the thyroid gland properly [1].
  • The level of TSH drops under the action of many hormones. Cortisol, whose level is elevated during a caloric restriction, plays an inhibiting role on the thyroid in lowering the level of TSH.
  • Other factors are raised during the diet prevent the transformation of T4 (inactive hormone) to the active hormone T3. This is the case with glucagon for example. The T4 is degraded without being transformed into T3 or is transformed into an inactive form of T3 called reverse T3 (rT3). A deficiency in protein increases this phenomenon.
  • Some metabolites resulting from the degradation of T3 reduce the rate of conversion of T4 to T3.
  • The level of free T3 lessens when it is tied to a binding protein. While this extends the lifetime of the T3, it also renders it inactive.
  • A puzzling phenomenon needs to be pointed out. Normally, GH raises the level of free T3. This is one of the reasons why GH increases basal metabolism and reduces fat. One could therefore be entitled to expect an elevation of the level of T3 during a diet since the concentration of GH is raised following a caloric restriction.
    In fact, it is the opposite which occurs. The level of free T3 drops in spite of a rise in the level of GH. Therefore, we must differentiate the effects of GH during a diet and when not on a diet:
  • GH with a normal or elevated caloric ration is essentially anabolic and increases basal metabolism.
  • GH during a caloric restriction is catabolic (for muscle and for fat) and is unable to oppose the lowering of the rate of T3.

These effects can appear paradoxical. They are explained by the increase or not of the level of IGF-1.

In conclusion: The macronutrients will have an important role in the stimulation, or the slowing of the thyroid [2]. Carbohydrates are the key macronutrient. They have a direct action on insulin and indirect on IGF-1, while inhibiting the secretion of glucagon. Therefore, The more you reduce carbohydrates during a diet and the longer your diet lasts, the more you will reduce the secretion of thyroid hormones. This is the big problem of diets without carbohydrates. They will drastically reduce the basal metabolism, hence the value of periodically resuming a normal (i.e. not low calorie or low carbohydrate) diet to restart the thyroid.

Consequences of the lowering of the activity of the thyroid

Lowering of the secretion of thyroid hormones seems disadvantageous during a diet. It translates not only to a lowering of the caloric expenditure but also to a reduction of lipolysis due to two phenomena: fewer beta receptors on the adipocytes and more potent factors preventing lipolysis. These factors are adenosine, phosphodiesterases, etc. As you can see, all this is very harmful for your fat reduction.

The solution seems easy: raise the activity of the thyroid. Unfortunately, the thyroid hormones also have a large impact on the muscles. As we have said, the thyroid hormones can be catabolic for the muscle. Insulin, if it is present in a sufficient quantity, can prevent the catabolism induced by the thyroid hormones. Therefore when one eats normally, the thyroid hormones are essentially anabolic. On the other hand, when the level of insulin is low due to caloric deficiencies, it is impossible to protect the muscle. Conclusion: during a diet, if the thyroid hormones were maintained at an elevated level, they would have essentially a catabolic action on the muscle. It is also one of the reasons why the level of thyroid hormones is reduced during a diet: preservation of the muscle mass. In consequence, people who take thyroid extracts rapidly lose mass during a diet. We must also add that the degradation of thyroid hormones is potentialized by cortisol, and this hormone is also increased by the diet [3]. The muscle catabolism due to thyroid hormones is both direct and indirect: the thyroid hormones take amino acids from the muscle while increasing and accelerating the catabolism of amino acids in the liver.

To summarize, it is important to preserve a normal activity of thyroid hormones during a diet, without wanting to increase their level excessively with thyroid extracts. In the next section we will see how to detect the lessening of activity of the thyroid with simple tests. This will permit us to optimize our diet while avoiding all reduction of basal metabolism. Moreover, I will write of Triac, a famous metabolite of T3.


  1. Burrow GN. 1990. Interaction of TSH, insulin and IGF in regulating thyroid growth and function. Growth Factors. 2:99. Medline.
  2. Van Gaal LF. 1989. Reverse T3 changes during protein supplemented diets: Relation to nutrient combustion rates. J Endo Invest. 12:799. Medline.
  3. Young VR. 1986. Synergism of triiodothyronine and corticosterone on muscle protein breakdown. Biochim Biophys Act. 883:106. Medline.

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