another:
Net protein synthesis is always a balance between the breakdown and accretion of muscle that is always occurring. T3 stimulates both, but in every study I have ever seen T3 administration either causes a net decrease in muscle mass, or no change.
One interesting thing about these studies is that the body seems to adapt to the T3. Initially there is net negative protein synthesis, but this tends toward baseline as the length of the study period increases. Here is a quote from one study that looked at this (they were using 50mcg/day)
"Frank hyperthyroidism is known to be associated with increased catabolism and loss of LBM (16). The present study suggests that the body may be able to partially compensate over time for the catabolic effects of more moderate increases in T3. Although both lean and fat mass were decreased by T3 treatment, as has been seen previously, N balance showed an initial decline, but returned to neutral and, in some individuals, even positive N balance by 9 weeks. This finding is similar to that of Wilson and Lamberts (17), who reported that T3 treatment in obese patients, while promoting weight loss, did not cause a deterioration in N balance. It is also similar to the few findings available from long term space flight, which suggest that astronauts partially compensate for the initial negative N balance in space (18)."
"During the first 3 weeks of T3 treatment, subjects exhibited a significantly negative N balance. Specifically, N balance during the second and third weeks of T3 treatment was significantly lower than the average baseline N balance (P < 0.05; Fig. 3). Subsequently, mean N balance tended to return toward zero, although five of the subjects remained in negative N balance for the duration of the study. There was a particularly large range of N balance values for the final week (week 9), with some individuals exhibiting quite negative (-37.8 g/day) and others quite positive (+11.1 g/day) N balance. Despite this variance, there was a significant difference between the pooled data from the second through fourth weeks of treatment compared to data from the fifth through ninth weeks (P = 0.002)."
"In summary, the present model of experimental hyperthyroidism indicates that healthy men are able to compensate over time for mild increases in serum T3 concentrations by decreasing N losses. There are persistent changes in lean and fat masses as well as measures of EE in this paradigm, however. The dose of thyroid hormone used did not produce clinically significant objective or subjective adverse effects in these young men despite accelerated catabolism."
J Clin Endocrinol Metab 1997 Mar;82(3):765-70
A paradigm of experimentally induced mild hyperthyroidism: effects on nitrogen balance, body composition, and energy expenditure in healthy young men.
Lovejoy JC, Smith SR, Bray GA, DeLany JP, Rood JC, Gouvier D, Windhauser M, Ryan DH, Macchiavelli R, Tulley R.
Pennington Biomedical Research Center, Louisiana State University, Baton Rouge 70808, USA.
lovejoj@mhs.pbrc.edu
Although T3 exerts major regulatory actions in both animals and humans, most clinical studies of T3 administration have been relatively short term. The present study examined the effects of more than 2 months (63 days) of low dose T3 treatment on nitrogen balance, body composition, 24-h energy expenditure (EE), and protein turnover in seven healthy men studied at an in-patient metabolic unit. Subjects were also randomly assigned to either high or low fat diets to determine the effects of diet composition. T3 treatment produced significant losses in both lean mass (1.5 +/- 0.3 kg) and fat mass (2.7 +/- 0.4 kg) by 6 weeks, with similar reductions in both at 9 weeks. The high fat diet somewhat attenuated the loss of body fat. Nitrogen balance was significantly negative for the first 3 weeks of T3 treatment, but tended to return to baseline thereafter. There were no significant effects of treatment on protein turnover at 9 weeks, although there was a slight increase in leucine oxidation (P = 0.07). Despite the apparent adaptation in nitrogen balance, total 24-h EE and sleeping EE were significantly increased at 9 weeks. We conclude that although healthy men are able to adapt to mild hyperthyroidism in terms of nitrogen balance, they exhibit significant and persistent changes in fat and fat-free mass as well as energy balance.
