Endocrine Abstracts

Introduction: Appropriately titrated levothyroxine (L-T₄) replacement therapy may not be able to fully correct the entire set of metabolic defects afflicting individuals with primary hypothyroidism. Based on this hypothesis, the present study was undertaken to establish whether these patients have impaired whole body energy metabolism.

Methods: We recruited 30 hypothyroid women with duration of the disease >2 years, BMI >25 kg/m² and serum TSH <3.5 μU/ml under replacement therapy with L-T₄ (mean dose: 73±34 μg/die) and compared them to 18 eu-thyroid women matched for age (53±13 vs 48±10 years), BMI (32.5±7.0 vs 33.7±8.3 kg/m²), menopausal state and life-style habits (P>0.3 for all). They underwent bioelectrical-impedentiometry (BIA) and indirect calorimetry to assess body composition and resting energy expenditure (REE) respectively.

Results: TSH (1.92±1.06 vs 1.87±0.89 μU/ml; P=0.91) and body composition (body fat: 41.4±7.4 vs 42.1±8.3%; LBM 58.6±7.4 vs 57.8±8.3%; P>0.7 for all) were not different between groups. REE was reduced in hypothyroid women when compared to the control group in absolute terms (1347±171 vs 1447±154 kcal/die; P<0.05), when adjusted for LBM (28.3±2.6 vs 30.5±3.0 kcal/kg LBM die; P<0.02) and when expressed as the ratio between the measured REE and the expected REE based on the Harris-Benedict Equation (91±7 vs 95±7%; P<0.05). The respiratory quotient was also different between groups (0.92±0.07 vs 0.86±0.06; P<0.01), suggesting for impaired fasting lipid oxidation in hypothyroid women.

Conclusions: This study demonstrates that middle-aged, overweight/obese hypothyroid women in L-T₄ replacement therapy, in spite of achieving an optimal serum TSH level, are characterized by altered whole body energy metabolism and substrate disposal supporting the view that additional interventions may be necessary to fully revert the entire set of hypothyroidism-related metabolic alterations.