—Before beginning our study on the use of LD1 for the diagnosis of myocardial infarction, we also encountered a patient whose condition was diagnosed as myxedema who had an elevated CK-MB level and an LD1/LD2 ratio flip, with a normal ECG and no chest pain. We did not have any additional serum remaining to test with the commercially available isoimmune kit (Isomune-LD). However, we would have anticipated an elevated LD1 concentration. It is therefore likely that hypothyroidism is another clinical entity yielding elevated LD1. Under ordinary circumstances, angina or myocardial infarction does not usually develop in patients with hypothyroidism; however, some patients may have chest pain and, indeed, even infarct, if replacement therapy is given too vigorously. In these situations, albeit uncommon, the finding of an elevated LD1 concentration should be interpreted with caution.