Lean body mass is a major determinant of levothyroxine dosage in the treatment of thyroid diseases

J Clin Endocrinol Metab. 2005 Jan;90(1):124-7. doi: 10.1210/jc.2004-1306. Epub 2004 Oct 13.


Total body weight is usually employed to calculate the amount of l-T(4) to be administered in patients with thyroid diseases. The aim of this study was to evaluate the effect of body composition on l-T(4) requirements. Body composition was assessed by dual energy x-ray absorptiometry in 75 patients on TSH-suppressive l-T(4) therapy after conventional thyroid ablation for differentiated cancer. The mean daily dose of l-T(4) was lower in normal-weight (127.5 +/- 21.3 mug/d) vs. overweight (139.4 +/- 24.5) and obese (151.3 +/- 29.1) subjects. There was a much stronger association between the l-T(4) dosage and lean body mass (P < 0.001, r = 0.667) compared with fat mass (P = 0.023, r = 0.26). Measurement of regional tissue composition showed peripheral lean mass as the best correlate with the dose of l-T(4) (r = 0.679, P < 0.001) whereas no correlation was observed with peripheral fat mass. In conclusion, individual l-T(4) requirements are dependent on lean body mass. Age- and gender-related differences in l-T(4) needs reflect different proportions of lean mass over the total body weight. An estimate of lean mass may be helpful to shorten the time required to attain a stable dose of l-T(4), particularly in subjects with high body mass index values that may be due either to increased muscular mass or to obesity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Body Composition*
  • Female
  • Humans
  • Leptin / blood
  • Male
  • Thinness
  • Thyroid Diseases / drug therapy*
  • Thyroid Diseases / metabolism
  • Thyrotropin / blood
  • Thyroxine / administration & dosage*


  • Leptin
  • Thyrotropin
  • Thyroxine