To the Editor. —In the JanuaryArchives, Klein and Levey 1 discussed unusual manifestations of hypothyroidism. In their discussion of musculoskeletal features, elevations of the levels of creatine kinase (CK) and lactate dehydrogenase (LDH) were mentioned as being observed in up to 80% of the patients with hypothyroidism. The authors remarked that determinations of normal percentages of CK-MB isozyme confirmed the musculoskeletal origin of CK in hypothyroidism and that elevations of LDH levels could reflect increased release and/or decreased clearance from the liver. No mention was made of possible cardiac sources of CK and LDH in hypothyroidism. Prior studies of LDH isozymes in myxedema heart disease have shown that LDH isozyme 1 (LDH 1 ) may be elevated in this disorder and gradually fade with thyroid replacement therapy. 2 In a prior Archives article on LDH, 3 I failed to recognize the association of hyperthyroidism with elevated levels of this isozyme.