Effect of L-thyroxine (LT4) and D-thyroxine (DT4) on cardiac function and high-energy phosphate metabolism: a 31P NMR study.

31P NMR spectroscopy was used to monitor the cardiac energy metabolism in hypothyroid rat hearts. Differential alterations in phosphocreatine and inorganic phosphate levels were observed upon treatment of hypothyroid animals with DT4 and LT4, while both agents were equipotent in reducing cholesterol. These results show potential for NMR spectroscopy as a technique to determine therapeutic selectivity.