Lean body mass is a major determinant of levothyroxine dosage in the treatment of thyroid diseases.
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.
Department of Endocrinology, University of Pisa, 56124, Pisa, Italy. fsantini@endoc.med.unipi.it
Ferruccio
F
Santini
Aldo
A
Pinchera
Alessandro
A
Marsili
Giovanni
G
Ceccarini
Maria Grazia
MG
Castagna
Rocco
R
Valeriano
Monica
M
Giannetti
Donatella
D
Taddei
Roberta
R
Centoni
Giovanna
G
Scartabelli
Teresa
T
Rago
Claudia
C
Mammoli
Rossella
R
Elisei
Paolo
P
Vitti
D001823:Body Composition; D005260:Female; D006801:Humans; D020738:Leptin / Q000097:blood; D008297:Male; D013851:Thinness; D013959:Thyroid Diseases / Q000188:drug therapy* / Q000378:metabolism; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000008:administration & dosage*
D020738:Leptin; D013972:Thyrotropin; D013974:Thyroxine