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Context: It is well established that subclinical hypothyroidism (SCH) is associated with mild cardiac dysfunction, but it is unknown whether there is an underlying impairment of cardiac bioenergetic function. Objective: The objective of the study was to quantify the cardiac phosphocreatine to adenosine triphosphate ratio (PCr to ATP) in SCH, compared with healthy controls, and to measure the effect of 6 months of levothyroxine treatment. Design and Setting: This was a 6-month, prospective, case-controlled interventional study. Participants and Main Outcome Measures: The PCr to ATP ratio was measured using phosphorus-31 magnetic resonance spectroscopy in subjects with SCH at baseline and after levothyroxine therapy (1.6 μg/kg·d) and compared with age- and gender-matched euthyroid controls. All subjects were free of overt heart disease. Results: Twenty-one subjects with SCH (normal free T4 and serum TSH between 4.1 and 10 mIU/L) and 17 controls were matched for age (mean age 40.5 vs 43.3 y) and sex (females 81% vs 82%) but differed in mean TSH (6.5 vs 2.1 mIU/L, P &amp;lt; .001). At baseline the mean (±SD) PCr to ATP ratio in SCH was lower than in controls (1.80 ± 0.26 vs 2.07 ± 0.20, P = .001). In the 16 subjects studied after levothyroxine treatment, the PCr to ATP ratio improved (from 1.74 ± 0.24 to 1.91 ± 0.26, P = .004) and approached controls (borderline loss of significance, P = .051). On multivariate analysis, SCH was independently associated with a reduced PCr to ATP ratio, even after adjusting for confounding variables (body mass index and fasting glucose) (P = .001). Conclusion: Our results demonstrate early cardiac bioenergetic impairment in SCH, which is reversible with levothyroxine therapy. This mechanistic insight provides justification for longitudinal trials to determine whether improvement in bioenergetic function improves cardiovascular outcome.

Levothyroxine Improves Abnormal Cardiac Bioenergetics in Subclinical Hypothyroidism: A Cardiac Magnetic Resonance Spectroscopic Study