In fact, exogenous T4 or T3 is fine. Some claim that exogenous T4 will be less suppressive on the thyroid. Truth is I see no evidence, but do see the facts that both T4 and T3 individually impart a negative feedback resoponse on the Hypothalmus-Pituitary-Thyroid Axis, decreasing TRH and TSH...
With T4 you are just letting your body determine how much additional T3 it can convert the surplus of T4 into...
Ahhh, the mysterious G3 receptor? j/k
Not true my friend. Although hormones do agonize a variety "non native" receptors (ex. hGH -->GHR & PRLR),
there is no "fight" between GH and T3.
T3's effects are mediated by TR (thyroid receptor)alpha and beta. GH's effects are mediated by the GHR as well as the GH mediated IGF-1 that agonizes IGF-1 receptors
T3 upregulates both GHR and IGF-1R mRNA which obviously makes GH more efficient at carring out its task.
GH on the otherhand is responsible for eventually lowering T3 levels (see my previous post). You can see how the relationship is structured. There is MUCH more to the Thyroid-GH/IGF-1 relationship than just this....
True. Iodothyronine Deiodinase Type 1 and 2 are responsible for the conversion of Thyroxine (T4) to Tri-iodothironine (T3).
Deiodinase Type 3 is responsible for conversion of T4 and T3 into inactive forms.
Sorry, I do not agree with this.
The actual issue (in the BB arena) started with an article by Anthony Roberts. Although many of his facts in the article were properly cited, his own conclusions were flawed in my opinion (and many others who understand the specifics).
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In summary, he concluded that increasing T3 (exogenously) in the presence of decreased T4, which occurs via the increased (GH mediated) peripheral T4 to T3 conversion, would lead to upregulated deiodinase Type 3 (D3) and downregulated D1 and D2. This was his basis for supporting that T3 would be ineffective comapared to T4. High D3 would "negate" T3's intended effects (as it would convert T3 to inactive metabolites). Actually, his way of stating his reasoning is much more bizaar as he states that it is the T4 to T3 conversion process that is responsible for the effects of T3 and NOT T3 in itself. This is the most bizaar statement in the whole article and defies the whole concept and target of Thyroid Hormone function.
In reality this is all much more complex than any of this and there are missing pieces in the conclusions above.
The fact is that the up and downregulation of deiodinases is only one piece and what is not taken into account is that both T4 AND T3 are targets (of "inactivation") of D3, NOT just T3 as Anthony proposes. This greatly changes things.
Even in spite of this proposed possibility, exogenous doses are "exogenous" and adjustible and ultimately not necessarily limited by any of the bodies endogenous adjustments...
The only thing I see in all this is that there really is not much difference in the end, between T4 or T3 supplementation. neither in protocol efficiency, nor in the suppression of the H-P-Thyroid Axis.
Nothing, at least, that I have seen concrete evidence of.
If anyone has any info that could enlighten me as to anything talked about here please feel free to chime in...
Take care.