To the Editor. —We read with interest the article by Klein and Levey, 1 the letter by Strasberg, 2 and the reply by Klein and Levey. 3 Dr Strasberg pointed out that an elevated lactate dehydrogenase isozyme 1 (LDH , ) level originated from cardiac muscle and "that hypothyroidism should be added to the list of etiologies associated with elevated levels of LDH , ." We wish to report our results relating to lactate dehydrogenase (LDH) isozyme levels in hypothyroidism, which support Klein and Levey's reply. In our recent study, 27 of 45 patients with hypothyroidism showed elevated total LDH levels. The LDH isozyme was measured in 22 of these 27 patients, and a normal pattern of LDH isozyme was found in 12 patients. In the remaining ten, three showed increased LDH 1 levels, one showed an increased lactate dehydrogenase isozyme 3 level, and six showed elevated lactate dehydrogenase isozyme 5 (LDH 5 ) levels.