Effect of thyrotoxicosis and recovery on myocardial protein balance.

Thyroid hormone, given in vivo or in vitro, exerts an anabolic effect on the heart. The hypertrophy that is produced by daily thyroxine injections in vivo is mediated primarily by an increase in protein synthesis. The changes in protein balance seem to be mediated at least in part by a direct action of thyroid hormone on heart cells, independent of secondary changes in hemodynamic or neurohumoral factors (although these might well contribute to the final effects in vivo, of course). The cardiac catabolism that accompanies regression of thyroxine-induced hypertrophy is characterized by a marked reduction in protein synthesis, not by an acceleration of protein breakdown.