Lean body mass as a predictor of levothyroxine requirement in primary hypothyroidism as compared to actual body weight

Abstract Background Levothyroxine (LT4) therapy for hypothyroidism is traditionally dosed at 1.6 μg/kg of actual body weight (ABW). However, ABW-based dosing often fails to proportionally adjust for increasing body weight. Ideal body weight (IBW) and lean body mass (LBM) have been proposed as alternatives, but the data on these parameters on larger hypothyroid population is lacking, particularly in Hashimoto's thyroiditis. This study evaluates LBM- and IBW-based LT4 dosing and examines variability in ABW-based dosing across age, BMI, and menopausal status. Methods This cross-sectional study analyzed 720 patients with primary hypothyroidism on stable LT4 doses and in a euthyroid state for ≥6 months. ABW, BMI, LT4 dose, and TSH were recorded. IBW and LBM were calculated using Devine’s and Boer’s formulas, respectively. LT4 doses per ABW, IBW, and LBM were compared across age, BMI, and menopause. Results Daily LT4 dose per kilogram of ABW decreased across BMI categories (18.5–24.9 kg/m2: 1.73±0.34 µg/kg, 25–29.9 kg/m2: 1.51±0.30 µg/kg, ≥30 kg/m2: 1.33±0.31 µg/kg; p0.001). In contrast, IBW-based dosing increased with BMI (1.92±0.40, 2.09±0.47, 2.25±0.51 µg/kg respectively; p0.001), while LBM-based dosing remained consistent (2.37±0.48, 2.37±0.52, 2.35±0.54 µg/kg respectively; p=0.91). LT4 dose decreased significantly with age (p0.001) in ABW-based dosing but showed no significant change with LBM (p=0.224) or IBW (p=0.377). ABW-based dosing was significantly lower in postmenopausal patients (p0.001), while IBW and LBM-based dosing showed no significant variation. Conclusion LT4 dosing based on LBM offers a more consistent approach for managing hypothyroidism. A dose of 2.3 mcg/kg LBM may optimize treatment outcomes.