How many times have you heard people talk about their thyroids? If you are into fitness, probably more times than most, and probably in the context of great frustration. A comment like, “My thyroid is so slow, “ is something you might commonly hear in physique-based sports. I’m not going to lie, some days I hate my thyroid. I mean, it’s that small, tiny thing that basically controls whether you can eat ice cream and be lean or eat salad and be large. To a point, most people know what a thyroid is. However, let’s dig a little bit deeper into understanding how it works and what can make it so annoying.

What Is the Thyroid?

Look down at your hands and place your two thumbs together. This is approximately the size of your thyroid. Despite its small stature, the thyroid is one of the largest endocrine organs in your body. Your thyroid is regulated by a series of feedback loops telling your metabolism when to go full speed and when to slow down. More specifically, it is responsible for producing hormones like calcitonin, triiodothyronine (T3), and thyroxine (T4).

Tyrosine and iodine are critical for the production of T4 and T3. To make these hormones, a long chain of linked tyrosine amino acids form a glycoprotein known as thyroglobulin (Tg), which acts as a substrate to synthesize both T41. Once synthesized, the thyroid gland then releases these hormones into circulation. Thyroid transport proteins then carry these hormones to specific tissues that contain intracellular thyroid hormone targets.

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A lot of people may have heard of T4, as it is responsible for 90% of thyroid hormone production. However T3, which makes up only 10% of the thyroid hormone production, is the real star when it comes to your metabolic rate. On an average day, a normal thyroid produces 100µg of T4 and 6µg of T32.  In addition, another 24µg of the T3 is made by extrathyroidal tissue T4 deionization. At this point you might be wondering, “Why is T3 the star of the thyroid show?” Well, for starters, it is four times as potent as its T4 counterpart. Additionally, T3 is the hormone that actually exerts action on mitochondrial and nuclear DNA. That's right; T3 is an important regulator of your metabolic fate because it actually regulates the expression of specific genes.


This T3 and T4 Business

Basically, T4 is Sparta (in the thyroid).  It DOMINATES thyroid hormone production by converting to either T3 or reverse T3 (rT3). T3 can be your friend when it comes to weight loss but your enemy when it comes to weight gain (muscle). The sympathetic nerves increase thyroid hormone release, as well as its association with weight gain and food intake. However, in response to stimuli like weight loss and fasting, sympathetic activity is suppressed, and rT3 (inactive thyroid hormone) is released1. Thus, there is a feedback loop in which catabolic events (fasting, weight loss) promote suppression of active thyroid hormone (T3) and anabolic events (like weight gain and eating) promote the opposite (to an extent).

Too much active and functional T3 can result in a huge spike in your metabolism and energy demands. If these demands are not met, your body goes into a state of catabolism that often results in the break down of muscle mass (not fat mass). In this catabolic state, your body searches for things to break down. Your body doesn’t think, “Oh hey, I want you to be super lean.” It also doesn’t go about preferentially eating up your fat stores. Instead, in most conditions, it metabolizes things that provide a lot of energy and also require a lot of energy to maintain. Unfortunately, muscle is one of those things. So, while a fast metabolism is great way to potentially demolish body weight, exogenously increasing thyroid speed can come at the price of some hard earned muscle. Important point: having a lot of T3 doesn’t mean you will burn up all your fat stores! In the game of homeostasis, where muscle is more expensive than fat (when it comes to energy dollars), T3 has a tendency to burn through muscle mass. And, as an elitefts reader, I’m sure that is the last thing you want.

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It should also be mentioned that there is one other hormone that is produced from T4, known as rT3. rT3 is not your friend when it comes to weight loss. In response to excess T4, your body produces rT3 to prevent all excess T4 from becoming converted to T3. Remember, our bodies are smart. They don’t want to give us an abundance of catabolic hormones like T3, because they know it can be harmful to the body’s balance between catabolism and anabolism. High levels of T4 are not the only thing that increase rT3 levels. Stress also increases the conversion of T4 to rT3. So, next time you start to get stressed, get that under control, because no one deserves to have a stressful day and gain adipose. That is a no-win scenario in the worst way!

Let Them Do Their Thing

Most people want to know how they can maximize the conversion of T4 to T3, without harming their metabolism.

First things first, let’s understand the dynamics of exogenous T3 and/or T4. In your fat tissue, supplementing with T3 or T4 inhibits Type II 5'-deiodinase (D2) activity3. That is unfortunate since D2 is responsible for that awesome conversation of T4 to T3. Thus, with exogenous thyroid hormone consumption, there is a risk of shutting down endogenous hormone production. Additionally, with exogenous T4 consumption there is no good way of knowing if that T4 will get converted into rT3 or T3 until after the fact. For most, levels of your own thyroid hormones are nicely tuned to promote a balance between lipogenic (fat forming) and lipolytic (breaking down fat) processes in the body. Attempting to shift that balance towards more lipolysis through exogenous supplementation may likely result in the crashing and burning of your metabolism. The thyroid is one powerful, little, hormone-secreting gland. It’s also a bit of a control freak. Accordingly, it is probably best to let it do its thing*. So, next time you hear, or are attempted to say, “my thyroid is so slow,” take a look at your life schedule. Get your diet balanced, get adequate sleep, and stress less. In the long run, that will probably be more helpful to getting your thyroid rolling and metabolism running than anything else.

*Hypothyroid conditions may require medical supervision and medical doctor prescribed supplementation.  If you are concerned that you have a thyroid disorder, you should seek the help of a medical professional. No medical advice is contained in this article.


  1. Borer, K.T. Exercise endocrinology, (Human Kinetics, Champaign, IL, 2003).
  2. Pilo, A., et al. Thyroidal and peripheral production of 3,5,3'-triiodothyronine in humans by multicompartmental analysis. The American journal of physiology 258, E715-726 (1990).
  3.  Calvo, R.M. & Obregon, M.J. Presence and regulation of D1 and D2 deiodinases in rat white adipose tissue. Metabolism: clinical and experimental 60, 1207-1210 (2011).