Additional T3 is not all that’s needed here. What’s needed is the actual conversion process of T4-T3, and the deiodinase presence and activity that it involves. This is because Local 5'-deiodination of l-thyroxine (T4) to active the thyroid hormone 3,3',5-tri-iodothyronine (T3) is catalyzed by the two 5'-deiodinase enzymes (D1 and D2). These enzymes not only "create" T3 out of T4, but actually regulates various T(3)-dependent functions in many tissues including the anterior pituitary and liver. So when there is an excess of T3 in the body, but normal levels of T4, the body’s thyroid axis sends a negative feedback signal., and produces less (D1 and D2) deiodinase, but more of the D3 type, which signals the cessation of the T4-T3 conversion process, and is inhibitory of many of the synergistic effects that T3 has! Remember, Type 3 iodothyronine deiodinase (D3) is the physiologic INACTIVATOR of thyroid hormones and their effects (13) and is well known to have independent interaction with growth factors (which is what GH and IGF-1 are).(14) This is because with adequate T4 and excess T3, (D1 and D2) deiodinase is no longer needed for conversion of T4 into T3, but levels of D3 deiodinase will be elevated. When there is less of the first two types of deidinase, it would seem that the T3 which has been converted to T4 can not exert it’s protein sparing (anabolic effects), as those first two types are responsible for mediation of many of the effects T3 has on the body. This seems to be one of the ways deiodinase contributes to anabolism in the presence of other hormones.
All of this would explain why anecdotally we see bodybuilders who use T3 lose a lot of muscle if they aren’t using anabolics along with it- they’re not utilizing the enzyme that would regulate some of T3’s ability to stimulate protein synthesis, while they are simultaneously signaling the body to produce an inhibitory enzyme (D3). And remember, for decades bodybuilders who were dieting for a contest have been convinced that you lose less muscle with T4 use, but that it’s less effective for losing fat when compared with T3? Well, as we’ve seen, without something (GH in this case) to aid in the conversion process, it would clearly be less effective! Since the deiodinase enzyme is also located in the liver, and we see decreased hepatic nitrogen clearance with GH + T3, it would seem that the D3 enzyme is exerting it’s inhibitory effects, but in the absence of the effects of the first two deiodinase enzymes, it remains unchecked and therefore not only limits the GH’s nitrogen retention capability.
In other words, if we have enough to GH in our body aid in supraphysiological conversion of T4 into T3, but we already have the too much (exogenous) T3, the GH is not going to be converting any excess T4 into T3 after a certain point- which would be a limiting factor in GH’s anabolic effects, when coupled with the act that we’ve allowed the D3 enzyme to inhibit the T3/GH synergy that is necessary.
As further evidence, when we look at certain types of cellular growth (the cartilage cell in this case) we see that GH induced rises in IGF-I stimulates proliferation, whereas T3 is responsible for hypertrophic differentiation. So it would seem that in some tissues, IGF-1 stimulates the synthesis of new cells, while T3 makes them larger. In this particular case, The fact that T4 and (D1) deiodinase is am active component in this system is noted by the authors. They clearly state (paraphrasing) that: "T4 is is converted to T3 by deiodinase (5'-DI type 1) in peripheral tissues…[furthermore]GH stimulates conversion of T4 to T3 , suggesting that some effects of GH may involve this pathway." The thing I want you to notice is that the authors of this paper state that the that the conversion PATHWAY is probably involved, and not the simple presence of T3. (15 )
Also, that same study notes that T3 has the ability to stimulates IGF-I and expression in tissues that whereas GH has no such effect (ibid).
So what are we doing when we add T3 to GH? We’re effectively shutting down the conversion pathway that is responsible for some of GH’s effects! And what would we be doing if we added in T4 instead of T3? You got it- we’d be enhancing the pathway by allowing the GH we’re using to have more T4 to convert to T3, thus giving us more of an effect from the GH we’re taking. Adding T4 into our GH cycles will actually allow more of the GH to be used effectively!
Remember, the thing that catalyzes the conversion process is the deiodinase enzyme. This is also why using low amounts of T3 would seem (again, anecdotally in bodybuilders) to be able to slightly increase protein synthesis and have an anabolic effect – they aren’t using enough to tell the body to stop or slow down production of the deiodinase enzyme, and hence .Although this analogy isn’t perfect, think of GH as a supercharger you have attached to your car…if you don’t provide enough fuel for it to burn at it’s increased output level, you aren’t going to derive the full effects. Thyroid status also may influence IGF-I expression in tissues other than the liver.So what we have here is a problem. When we take GH, it lowers T3 levels…but we need T3 to keep our GH receptor levels optimally upregulated. In addition, it’s suspected that many of GH’s anabolic effects are engendered as a result of production of IGF-1, so keeping our IGF receptors upregulated by maintaining adequate levels of T3 seems prudent. But as we’ve just seen, supplementing T3 with our GH will abolish Growth Hormone’s functional hepatic nitrogen clearance, possibly through the effect of reducing the bioavailability of insulin-like growth factor-I (12.)
So we want elevated T3 levels when we take GH, or we won’t be getting ANYWHERE NEAR the full anabolic effect of our injectable GH without enough T3. And now we know that not only do we need the additional T3, but we actually want the CONVERSION process of T4 into T3 to take place, because it’s the presence of those mediator enzymes that will allow the T3 to be synergistic with GH, instead of being inhibitory as is seen when T3 is simply added to a GH cycle. And remember, we don’t only want T3 levels high, but we want types 1 and 2 deiodinase to get us there- and when we take supplemental T3, that just doesn’t happen…all that happens is the type 3 deiodinase enzyme shows up and negates the beneficial effects of the T3 when we combine it with GH.
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