Thyroid hormones are hormones that are secreted by the thyroid. The thyroid is an endocrine gland in the front of your neck that’s located directly below the larynx (Adam’s apple), weighing close to 20 g. The two principal thyroid hormones that it secretes are triiodothryonine (T3) and thyroxine (T4). The latter functions mostly as a prohormone as most of its effects rely on conversion into T3. This T4-to-T3 conversion, also called outer ring deiodination, takes place mainly outside of the thyroid in peripheral tissues. Collectively this leads to a daily production of about 88 mcg (113 nmol) T4 and 28 mcg (43 nmol) T3 [2]. About one fifth of the T3 is derived from the thyroid, whereas the other four fifths is produced by extrathyroidal T4-to-T3 conversion [3].
Just as with anabolic steroids, thyroid hormones are transported in the bloodstream by carrier proteins. The majority is bound to thyroxine-binding globulin (TBG), with the remainder bound to transthyretin, albumin and some lipoproteins. Collectively they bind over 99 % of thyroid hormones in the circulation. It’s thought that the unbound fraction is available for tissues for uptake and is responsible for its effects [4]. Though there are some caveats to the evidence for this, I’m not gonna get into a free hormone hypothesis discussion here (other than mentioning that in its strict form it’s wrong, but free thyroid hormone measurements are useful nevertheless).
Once it reaches the peripheral tissues, and crosses the plasma membrane of a cell, it’s time for some action. In the case of T4, it first needs to be converted into T3 as mentioned before, as T4 can be viewed as a prohormone. This conversion takes place inside the cell, either close to the plasma membrane (after which it quickly equilibrates with the blood plasma), or close to the cell nucleus—the site of action [5].T3, on the other hand, can directly continue its journey by getting into the cell nucleus. The cell nucleus is the organelle of the cell where gene transcription takes place. Just as anabolic steroids do, thyroid hormones primarily exert their effects through modulation of gene transcription. They do so by binding to thyroid hormone receptors which are mainly located inside the cell nucleus, bound to the DNA.
Thyroid hormones affect a vast array of tissues and have a plethora of effects, but for this article I’ll be focusing on its effect on energy metabolism and (skeletal muscle) protein turnover. Most likely the two most interesting for the people reading this in regard to its efficacy.
