title: Body composition and metabolic parameters are associated with variation in thyroid hormone levels among euthyroid young men.
abstract: OBJECTIVE
  Thyroid disorders affect metabolism and body composition. Existing literature has been conflicting on whether this is also the case for thyroid hormone levels within the euthyroid range. Therefore, we have investigated the relationship between thyroid hormone concentrations and body composition together with metabolic parameters in a population of healthy euthyroid men.
  
  
  METHODS
  Healthy male siblings (n=941, 25-45 years, median BMI 24.6) were recruited in a cross-sectional, population-based study; a history or treatment of thyroid disease and thyroid autoimmunity were exclusion criteria. Body composition and muscle cross-sectional area were assessed by dual-energy X-ray absorptiometry and peripheral quantitative computed tomography. Total (triiodothyronine (T(3); TT(3)) thyroxine and (T(4); TT(4))) and free thyroid hormones (FT(3) and FT(4)), TSH, and reverse T(3) (rT(3)) and thyroid-binding globulin (TBG) were determined using immunoassays.
  
  
  RESULTS
  BMI was positively associated with (F)T(3) (P<0.0001). Whole body fat mass displayed positive associations with TT(3) and with (F)T(4) and TBG (P≤0.0006). Positive associations were further observed between leptin and (F)T(3), TT(4), and TBG (P≤0.0002). Inverse associations between lean mass and muscle cross-sectional area and (F)T(3), (F)T(4), and TBG were observed (P≤0.0003). Higher levels of (F)T(3) and TBG were associated with lower insulin sensitivity, assessed by homeostatic model assessment of insulin resistance (IR; P≤0.0001). No associations between TSH and body composition or metabolic parameters were seen.
  
  
  CONCLUSIONS
  We show that a less favorable body composition (with higher fat and lower muscle mass and accompanying higher leptin concentrations) and IR are associated with higher thyroid hormone levels in healthy young men with well characterized euthyroidism.
authors.affiliation: Department of Endocrinology and Metabolic Bone Diseases, Ghent University Hospital, Belgium. greet.roef@ugent.be
authors.forename: Greet
authors.initials: G
authors.lastname: Roef
authors.forename: Bruno
authors.initials: B
authors.lastname: Lapauw
authors.forename: Stefan
authors.initials: S
authors.lastname: Goemaere
authors.forename: Hans-Georg
authors.initials: HG
authors.lastname: Zmierczak
authors.forename: Kaatje
authors.initials: K
authors.lastname: Toye
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Kaufman
authors.forename: Youri
authors.initials: Y
authors.lastname: Taes
mesh_terms: D000273:Adipose Tissue; D000328:Adult; D015415:Biomarkers / Q000097:blood; D001823:Body Composition; D015992:Body Mass Index; D002352:Carrier Proteins / Q000097:blood; D015986:Confounding Factors, Epidemiologic; D003430:Cross-Sectional Studies; D004958:Estradiol / Q000097:blood; D006801:Humans; D007333:Insulin Resistance; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D020738:Leptin / Q000097:blood*; D008297:Male; D008565:Membrane Proteins / Q000097:blood; D008875:Middle Aged; D012738:Sex Hormone-Binding Globulin / Q000378:metabolism; D013739:Testosterone / Q000097:blood; D013954:Thyroglobulin / Q000097:blood; D013963:Thyroid Hormones / Q000097:blood*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D013975:Thyroxine-Binding Proteins / Q000378:metabolism; D014284:Triiodothyronine / Q000097:blood; D000097810:Thyroid Hormone-Binding Proteins
chemical_list: D015415:Biomarkers; D002352:Carrier Proteins; D004958:Estradiol; D018972:Insulin-Like Growth Factor Binding Protein 3; D007334:Insulin-Like Growth Factor I; D020738:Leptin; D008565:Membrane Proteins; D012738:Sex Hormone-Binding Globulin; D013739:Testosterone; D013954:Thyroglobulin; D013963:Thyroid Hormones; D013972:Thyrotropin; D013974:Thyroxine; D013975:Thyroxine-Binding Proteins; D014284:Triiodothyronine; D000097810:Thyroid Hormone-Binding Proteins; C515497:IGFBP3 protein, human