As mentioned at the beginning of this article, a major roadblock in the adoption of T3 by the medical community as an antiobesity agent is its deleterious effect on the heart.
Recent research has identified two isoforms of the thyroid hormone receptor, TRalpha and TRbeta. The TRalpha-form may preferentially regulate the heart rate, and an experimental agent, GC-1, has been developed that selectively binds the TRbeta receptor, with minimal effects on the heart (30). The distribution and actions of TRalpha and TRbeta throughout the body are not yet well characterized. However should it turn out that TRalpha is specific to the heart, then drugs like GC-1 may turn out to be effective fat burning agents with a much safer profile that T3 or T4.
One alleged “futuristic” agent that is here now is T2,
or 3,5-Di-iodo-L-thyronine, the T3 metabolite discussed above. Unfortunately, this product does not live up to its hype.
It has been claimed to be as or more effective that T3 for fat burning with minimal suppression of endogenous thyroid production. Regarding the relative effectiveness of T2 as a lipolytic agent, and its effect on TSH, this topic was thoroughly covered in a recent article by Bryan Hay**** in Muscle Monthly:
All of my research into this subject has led me to the same conclusion reached by Mr. Hay****. That is, T2 is only slightly less suppressive of TSH than is T3, and only packs a portion of the lipolytic punch of T3, with no ability to increase the expression of the UCPs, which is a major determinant of the action of thyroid hormone. Summary
We have discussed a number of ways by which T3, and its active metabolite T2 act to increase resting energy expenditure. Also discussed were some drawbacks of T3 use, such as cardiac stress, as well as the potential loss of muscle mass.
It is ironic that the latter may be of more concern to many bodybuilders that the other more serious potential impacts on health. Nevertheless, used moderately and for short periods (a couple of months or less) in people with no preexisting cardiovascular disease T3 has a relatively safe medical profile, compared to other lipolytic agents like DNP. Perhaps most importantly we have presented substantial evidence that even the long-term use of supraphysiological levels of T3 does not damage the thyroid gland. References:
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