below is quote:
3,3'-Diiodothyronine
Advantages
* Almost no suppression of TSH/hypothalamic-pituitary-thyroid axis
* Stimulates cytochrome c oxidase - aka decouples Electron Transport Chain (slightly more than 3,5-T2)
* Precursor to T3 & T4 (which have their own metabolic properties of course)
Disadvantages
* Interferes with plasma membrane transport of T3
* No significant increases in growth hormone seen at any dosing levels
3,5-Diiodothyronine
Advantages
* Increases serum levels of growth hormone (GH) comprable to T3
* Stimulates cytochrome c oxidase - aka decouples Electron Transport Chain (slightly less than 3,3'-T2)
* Does not interfere with plasma membrane transport of T3
* Increases activation of Glucose-6-Phosphate Dehydrogenase
Disadvantages
* Suppression of TSH/hypothalamic-pituitary-thyroid axis is only slightly less but comprable to that of T3
Summary
If one is looking for strictly an increase in energy expenditure with less TSH depression, 3,3'-T2 is your answer..... If one is looking for more muscle mass preservation properties aswell and doesn't mind some TSH depression, 3,5-T2 is your answer....
On a final note though, I think T3 supplementation would probably be better than 3,5-T2 supplementation as serum 3,5-T2 as well as 3,3'-T2 concentrations have been shown to rise significantly with a marked rise in serum T3 following T3 administration. Also, just because T3 acts on the protein synthesis mechanism involved in the regulation of the mitochondrial mass (so is more indirect pathway) while both T2 act directly at the mitochondrial level, this doesn't mean it is less effective at interupting the ETC, in fact, T3 has been shown to exhibit slightly more of an effect even though it acts indirectly on the mitochondria....
