Administration of sodium L-thyroxine (LT 4), 0.4 mg/per day or more, was associated with decreases in the high serum CPK levels frequently present in the earlier stages of pseudohypertrophic muscular dystrophy. This was not observed during the administration of tenfold greater molar quantities of sodium D-thyroxine (DT 4). These data suggest that an L-configuration of this iodinated amino acid is essential for lowering the serum CPK. High dosages of a thyromimetic compound, an iodinated thyroalkanoic acid, also lowered serum CPK levels. LT 4 and DT 4 in the dosages employed produced comparable increases in pulse rate and each exerted a hypocholesterolemic effect. Hence, the lowering of CPK effected by LT 4 is mediated through mechanisms other than those which affect the cardio-vascular system and the serum cholesterol levels.