;)

Andregenic Receptor Modulation


Administration of T3 has been shown to upregulate the so-called beta 2 adrenergic receptor in fat tissue. What is the significance of this effect for fat loss? Before fat can be used as fuel, it must be mobilized from the fat cells where it is stored. An enzyme called Hormone Sensitive Lipase (HSL) is the rate-controlling enzyme in lipolysis, or fat mobilization. The body produces two catecholamines, epinephrine and norepinephrine, which bind to the beta 2 receptor and activate HSL. The upregulation of the beta 2 receptor due to T3 results in an increased ability of catecholamines to activate HSL, leading to increased lipolysis.

Bodybuilders often use drugs like clenbuterol, which bind to the beta 2 receptors and activate them in the same way as the body's endogenous catecholamines. The use of clenbuterol along with T3 can produce an additive lipolytic effect: T3 increases the number of receptors, while clenbuterol binds to the receptors activating HSL and increasing lipolysis. Since clenbuterol itself downregulates the beta 2 receptor, most bodybuilders use clenbuterol in a two week on/ two week off cycle, the rationale being that this minimizes downregulation and allows receptor recovery. Another option is to use the antihistamine ketotifen concurrently with the clenbuterol. Studies have shown that ketotifen attenuates the beta 2 receptor downregulation caused by clenbuterol (15). Moreover, research in AIDS patients has shown that ketotifen blocks the production of the proinflammatory and catabolic cytokine TNF-alpha (16). This may be of relevance to bodybuilders since there is evidence showing TNF lowers both testosterone and IGF-1 levels quite significantly (17) (18), while strenuous exercise elevates TNF levels. (19)

Besides increasing beta 2 receptor density in adipose tissue, T3 upregulates this receptor in human skeletal muscle (12). This has some very intriguing if somewhat speculative implications for the combined use of clenbuterol and T3. Animal studies have shown that catecholamines, particularly clenbuterol, inhibit Ca++ dependent skeletal muscle proteolysis (20). Like the lysosomal and ubiquitin-proteasome pathways discussed above, Ca++ regulated proteolysis is yet another way for the body to degrade muscle protein. Again the implications are enticing: Increased beta 2 receptor density from T3 use, coupled with the beta 2 agonist clenbuterol, could slow this pathway of muscle catabolism.

Another adrenergic receptor important to lipolysis is the alpha 2 receptor, which impedes fat mobilization by counteracting the effects of the beta 2 receptor. There are some conflicting studies about the effects of T3 on the alpha 2 receptor, with studies showing either a downregulation (21) or no effect (22). If T3 does in fact downregulate alpha 2 receptors, this would further aid lipolysis.

Studies in rats have shown that inducing hyperthyroidism increases the lipolytic beta 3 receptor density in white adipose tissue by 70% (23). Beta 3 receptors are abundant in human white adipose tissue as well, and if T3 administration has the same effect in humans, this could could contribute significantly to T3 induced fat loss. This might also argue for taking a currently available beta 3 agonist such as octopamine along with T3 and perhaps clenbuterol.



Decreased Phosphodiesterase Expression


In hyperthyroid patients as well as in normal subjects given T3, levels of the enzyme phosphodiesterase are lowered in fat cells (20). When lipolytic hormones like epinephrine (adrenaline) bind to the beta 2 receptor described above, they initiate a signaling cascade mediated by the so called “second messenger” cyclic AMP (cAMP). cAMP in turn acts on other cellular enzymes to initiate and maintain lipolysis. The original signal is terminated when cAMP is degraded by the enzyme phosphodiesterase. Clearly, maintaining elevated cAMP levels, by lowering phosphodiesterase concentrations with T3, will prolong lipolysis.

As an aside, caffeine is thought to exert at least a portion of its lipolytic action by lowering phosphodiesterase in fat cells. Interestingly, Viagra and Cialis are also phosphodiesterase inhibitors but their action seems to be limited to relaxing vascular smooth muscles.



Increased Growth Hormone Secretion


In vitro, animal, and human studies have all demonstrated that T3 administration increases growth hormone production. (24)(25) Since GH is calorigenic aside from any increase in igf-1, elevated GH may contribute to some of the fat burning associated with T3 administration. This effect may obviate the need for the use of expensive recombinant HGH, as mentioned above.



Decreased Insulin Secretion


Insulin is well known as a lipogenic hormone. It promotes fat storage by facilitating the uptake of fatty acids by adipocytes, and reducing lipid oxidation in muscle tissue. Several studies have shown that thyroid hormone is associated with glucose intolerance resulting from decreased glucose stimulated insulin secretion (26).

This defect in insulin secretion is believed to result from an increase in the rate of apoptosis (programmed cell death) of pancreatic beta cells as a direct effect of thyroid hormone excess.(27) This process is reversible, since when thyroid hormone is withdrawn the rate of beta cell replication increases until homeostasis returns. However, there are conflicting studies regarding the effects of T3 on insulin. For example, Dimitriadis et al (28) showed a decrease in glucose stimulated insulin secretion, consistent with (25), but an increase in basal insulin. They also observed increased insulin clearance, with a compensatory increase in basal insulin secretion.

So if in fact the hyperthyroid state is associated with lower insulin levels, this could explain a portion of hyperthyroid stimulated lipolysis. The obvious downside here is that insulin is also an anabolic hormone. Basal insulin concentration is thought to limit the action of the ubiquitin-proteasome degradative pathway of muscle protein breakdown (29). Of course supplementing with insulin during T3 use would be counterproductive. However, as mentioned above, anabolic steroids inhibit ubiquitin-proteasome activity, so their use could counter any loss in muscle anabolism resulting from a drop insulin levels.

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