Muscle metabolism and exercise tolerance in subclinical hypothyroidism: A controlled trial of levothyroxine

Background: Neuromuscular symptoms and impaired muscle energy metabolism have been described in subclinical hypothyroidism (sHT). Aim: The aim of the study was to evaluate the energy and substrate response to exercise in sHT patients using a standardized protocol and to test the effect of L-T-4 replacement in a double-blind, randomized, placebo-controlled fashion. Patients and Methods: We studied 23 sHT patients and 10 matched euthyroid controls. Oxygen uptake (VO2), carbon dioxide output, and heart rate were measured during incremental step-up exercise. Blood glucose, lactate, pyruvate, free fatty acid, glycerol, and beta-hydroxybutyrate concentrations were measured at rest, every 2 min during exercise, and during 20 min of recovery. The exercise protocol was repeated after 6 months of placebo or L-T-4-restored euthyroidism. Results: Maximal power output ( P = 0.02) and VO2 max ( P = 0.04) were reduced in sHT, and, with increasing workload, patients achieved higher heart rates ( P < 0.03) at VO2 values equivalent to those of controls. The respiratory quotient increments were significantly higher in patients than controls ( P < 0.04). Blood lactate and pyruvate and their ratio rose with a steeper slope ( P < 0.0001, P < 0.001, and P < 0.01, respectively) in patients than controls. Resting plasma free fatty acid and blood glycerol levels were significantly higher in patients than controls ( P < 0.0003 and P < 0.003, respectively) throughout baseline, exercise, and recovery. L-T-4 replacement, while improving neuromuscular symptoms, did not produce significant changes in the energy or substrate response to exercise. Conclusions: The response to exercise is altered both in terms of tolerance and pattern of substrate utilization in sHT patients. Restoring stable euthyroidism does not correct this defect over a 1-yr period.