Growth hormone and thyroid hormones have been standard drugs in the arsenals of bodybuilders for years. Growth hormone has been used for its anabolic, muscle cell increasing, fat loss and anti aging properties. Thyroid hormones have been used for the fat loss, stimulatory and (to a lesser extent) anabolic effects. In this article I will look at the synergies between the two different types of thyroid hormone (T4 and T3) and Growth hormone (GH) and their applications in bodybuilding.

Before we get into the juicy stuff, we need to start with a little ‘geek-talk’ and some physiology.

The body produces two thyroid hormones, the first is thyroxine (T4) and the second is triiodothyronine (T3), which is the most widely used thyroid hormone in the world of muscle building. T4 is the inactive thyroid hormone and needs to be converted to T3 to exert thyroid-specific effects. This is done by the enzymes in the deiodinase group, of which there are 3 types – D1 and D2, which involved in the initiation of the process of conversion of T4 to T3 and D3, which is involved in the deactivation process.

The secretion of T4 is created in the thyroid gland and is stimulated by Thyroid stimulating hormone (TSH), which in turn is stimulated by Thryrotropin Releasing hormone (TRH). So, when T3 levels rise, the body says, ‘hey, I have enough T3 floating around, so I need to cut back’, which it does by suppressing TSH (this is known as a ‘negative feedback loop’). Incidentally, thyroid hormones require insulin or IGF-1 to trigger their effects.

Growth hormone (GH) is produced in the pituitary gland and is regulated by factors such as hormones and enzymes. It is regulated by two hormones Somatostatin (SS) and Growth hormone releasing hormone (GHRH). When there is too much GH circulating the body another negative feedback loop tells it to produce SS to decrease GH levels. When the body has too little GH, GHRH is produced.

GH has the ability to stimulate the conversion of T4 to T3, making thyroid hormones partially dependent on GH. Somatostatin (which is secreted when GH levels are too high) can also inhibit TSH secretion or reduce TRH secretion, which means it can limit the amount of T4 produced by the body. This means that although GH increases the conversion of T4 to T3, which means more T3, it may actually mean lower than normal T4 levels.

GH gene transcription is what gives GH its wonderful effects (such as muscle growth, fat loss etc) and T3 enhances these effects, making GH and T3 extremely synergistic, in fact, T3 is the limiting factor in exogenous GH usage. Here we now have a contradiction: T3 and GH are synergistic, but too much T3 decreases the anabolic effects of GH.

This is where a well known steroid and performance enhancing drugs expert hypothesised that it is the conversion process of T4 to T3 that is important. Let me explain. When there is too much T3 in the body and normal levels of T4, the thyroid sends a signal to produce less D1 and D2 (the activators) and more of D3 (the in-activator) and thus inhibits many of the synergistic effects of T3. When D3 levels are high, growth factors such as IGF-1 are stimulated, which means D3 is an important part of the equation with regards to the anabolic effects of GH.

Now for the part you have all been waiting for, the summary and conclusion…

When growth hormone is taken, along with T3, the GH will stop converting T4 to T3 after a certain point, which means it will shut of the good, anabolic effects of GH by killing the pathway that creates them! This, to me, seems like a bad thing! Now if we add T4 into a GH cycle, we would enhance this pathway, giving the GH more anabolic effects!

Remember that T4 alone is pretty ineffective for our purposes and requires something like GH to be made effective.

I have actually seen how effective using T4 in conjunction with GH is, first hand. An athlete I was working with was dieting for a show using GH and T3 (along with many other anabolic compounds), but was seeing no change in his fat to muscle ratio. Even with changes in his diet, such as lowering carbs, increasing cardio and even lowering calories, there were minimal changes, at best. T4 was introduced and within 1 week a change of 2.5% body fat and an increase of 1kg of muscle was measured! This is a pretty drastic change in any book.

The way I see it, growth hormone costs a lot, and, if it were me, I would want to maximise it as much as possible to get a better ‘bang for my buck’, T4 seems to be the catalyst here and without it, GH is a waste of money.