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.
The Future
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 Haycock in Muscle Monthly:
All of my research into this subject has led me to the same conclusion reached by Mr. Haycock. 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:
(1) Endocrinology 2002 Feb;143(2):504-10 Are the effects of T3 on resting metabolic rate in euthyroid rats entirely caused by T3 itself? Moreno M, Lombardi A, Beneduce L, Silvestri E, Pinna G, Goglia F, Lanni A.
(2)(Greer,M. N Engl J Med 244:385, 1951)
(3)N Engl J Med 1975 Oct 2;293(14):681-4 Recovery of pituitary thyrotropic function after withdrawal of prolonged thyroid-suppression therapy. Vagenakis AG, Braverman LE, Azizi F, Portinay GI, Ingbar SH.
(4) J Clin Endocrinol Metab 1975 Jul;41(1):70-80 Patterns off recovery of the hypothalamic-pituitary-thyroid axis in patients taken of chronic thyroid therapy. Krugman LG, Hershman JM, Chopra IJ, Levine GA, Pekary E, Geffner DL, Chua Teco GN
(5) Int J Obes 1983;7(2):123-31 The effect of a low-calorie diet alone and in combination with triiodothyronine therapy on weight loss and hypophyseal thyroid function in obesity. Koppeschaar HP, Meinders AE, Schwarz F.
(6) Am J Med 2002 Jul;113(1):30-6 Effect of 6-month adherence to a very low carbohydrate diet program. Westman EC, Yancy WS, Edman JS, Tomlin KF, Perkins CE.
(7) J Endocrinol Invest 2001 Dec;24(11):897-913 Control of energy metabolism by iodothyronines.
Lanni A, Moreno M, Lombardi A, de Lange P, Goglia F
(
Clin Endocrinol (Oxf) 1980 Nov;13(5):489-506 Metabolic aspects of the calorigenic effect of thyroid hormone in mammals. Sestoft L.
(9)Annu Rev Nutr 1995;15:263-91 Thermogenesis and thyroid function. Freake HC, Oppenheimer JH.
(10) J Clin Invest 2001 Sep;108(5):733-7 Effect of triiodothyronine on mitochondrial energy coupling in human skeletal muscle. Lebon V, Dufour S, Petersen KF, Ren J, Jucker BM, Slezak LA, Cline GW, Rothman DL, Shulman GI.
(11)J Clin Endocrinol Metab 1999 Jan;84(1):207-12 Testosterone administration preserves protein balance but not muscle strength during 28 days of bed rest. Zachwieja JJ, Smith SR, Lovejoy JC, Rood JC, Windhauser MM, Bray GA.
(12) Genome Res 2002 Feb;12(2):281-91 In vivo regulation of human skeletal muscle gene expression by thyroid hormone. Clement K, Viguerie N, Diehn M, Alizadeh A, Barbe P, Thalamas C, Storey JD, Brown PO, Barsh GS, Langin D.
(13) J Clin Endocrinol Metab 2003 Jan;88(1):358-62 Related Articles, Links Differential anabolic effects of testosterone and amino Acid feeding in older men. Ferrando AA, Sheffield-Moore M, Paddon-Jones D, Wolfe RR, Urban RJ.
(14) J Hepatol 1996 Mar;24(3):313-9 Effects of long-term growth hormone (GH) and triiodothyronine (T3) administration on functional hepatic nitrogen clearance in normal man. Wolthers T, Grofte T, Moller N, Vilstrup H, Jorgensen JO.
(15) Cardiovasc Res 1998 Oct;40(1):211-22 Terbutaline-induced desensitization of human cardiac beta 2-adrenoceptor-mediated positive inotropic effects: attenuation by ketotifen. Poller U, Fuchs B, Gorf A, Jakubetz J, Radke J, Ponicke K, Brodde OE.
(16) Eur J Clin Pharmacol 1996;50(3):167-70 Ketotifen in HIV-infected patients: effects on body weight and release of TNF-alpha. Ockenga J, Rohde F, Suttmann U, Herbarth L, Ballmaier M, Schedel I.
(17)Endocrinology 1998 Jun;139(6):2863-8 Tumor necrosis factor-alpha inhibits leydig cell steroidogenesis through a decrease in steroidogenic acute regulatory protein expression. Mauduit C, Gasnier F, Rey C, Chauvin MA, Stocco DM, Louisot P, Benahmed M.
(18) Growth Horm IGF Res 2001 Aug;11(4):250-60 Tissue-specific regulation of IGF-I and IGF-binding proteins in response to TNFalpha. Lang CH, Nystrom GJ, Frost RA.
(19) Exerc Immunol Rev 2001;7:18-31 Exercise and cytokines with particular focus on muscle-derived IL-6. Pedersen BK, Steensberg A, Fischer C, Keller C, Ostrowski K, Schjerling P.
(20) Am J Physiol Endocrinol Metab 2001 Sep;281(3):E449-54 Catecholamines inhibit Ca(2+)-dependent proteolysis in rat skeletal muscle through beta(2)-adrenoceptors and cAMP. Navegantes LC, Resano NM, Migliorini RH, Kettelhut IC
(21) J Clin Endocrinol Metab 2002 Feb;87(2):630-4 Regulation of human adipocyte gene expression by thyroid hormone Viguerie N, Millet L, Avizou S, Vidal H, Larrouy D, Langin D.
(22) Metabolism 1987 Nov;36(11):1031-9 Alpha 2- and beta-adrenergic receptor binding and action in gluteal adipocytes from patients with hypothyroidism and hyperthyroidism. Richelsen B, Sorensen NS
(23) Br J Pharmacol 2000 Feb;129(3):448-56 Regulation of beta 1- and beta 3-adrenergic agonist-stimulated lipolytic response in hyperthyroid and hypothyroid rat white adipocytes. Germack R, Starzec A, Perret GY
(24) Braz J Med Biol Res 1994 May;27(5):1269-72 Role of thyroid hormone in the control of growth hormone gene expression. Volpato CB, Nunes MT.
(25) Am J Physiol 1999 Aug;277(2 Pt 1):E370-9 Related Articles, Links Low-dose T(3) improves the bed rest model of simulated weightlessness in men and women. Lovejoy JC, Smith SR, Zachwieja JJ, Bray GA, Windhauser MM, Wickersham PJ, Veldhuis JD, Tulley R, de la Bretonne JA.
(26) Life Sci 2002 Jul 19;71(9):1059-70 Evidence for a deficient pancreatic beta-cell response in a rat model of hyperthyroidism. Fukuchi M, Shimabukuro M, Shimajiri Y, Oshiro Y, Higa M, Akamine H, Komiya I, Takasu N.
(27) Diabetologia 2002 Jun;45(6):851-5 Thyroxine induces pancreatic beta cell apoptosis in rats.
Jorns A, Tiedge M, Lenzen S.
(28) Am J Physiol 1985 May;248(5 Pt 1):E593-601 Effect of thyroid hormone excess on action, secretion, and metabolism of insulin in humans.= Dimitriadis G, Baker B, Marsh H, Mandarino L, Rizza R, Bergman R, Haymond M, Gerich J
(29) Curr Opin Clin Nutr Metab Care 2000 Jan;3(1):67-71 Effects of insulin on muscle tissue.
Wolfe RR.
(30) J Steroid Biochem Mol Biol 2001 Jan-Mar;76(1-5):31-42 Selective modulation of thyroid hormone receptor action. Baxter JD, Dillmann WH, West BL, Huber R, Furlow JD, Fletterick RJ, Webb P, Apriletti JW, Scanlan TS.