Thyroid hormone status within the physiological range affects bone mass and density in healthy men at the age of peak bone mass

Context: The hormonal factors involved in the regulation of peak bone mass (PBM) in men have not been fully investigated. Apart from gonadal steroids and somatotropic hormones, thyroid hormones are known to affect bone maturation and homeostasis and are additional candidate determinants of adult bone mass. Objective: We aimed to investigate between-subject physiological variation in free and total thyroid hormone concentrations, TSH, and thyroid binding globulin (TBG) in relation to parameters of bone mass, geometry, and mineral density in healthy men at the age of PBM. Design and setting: We recruited 677 healthy male siblings aged 25-45 years in a cross-sectional, population-based study. Areal and volumetric bone parameters were determined using dual-energy x-ray absorptiometry (DXA) and peripheral quantitative computed tomography (pQCT). Total and free thyroid hormones, TBG, and TSH were determined using immunoassays. Results: Free and total thyroid hormone concentrations were inversely associated with bone mineral density (BMD) and bone mineral content (BMC) at the hip and total body (free triiodothyronine (FT3), total T-3 (TT3), and total T-4 (TT4)) and at the spine (FT3). TBG was negatively associated with BMC and areal BMD at all sites. At the radius, cortical bone area was inversely associated with TT3, TT4, and TBG, and trabecular bone density was inversely associated with free thyroxine, TT4, and TBG. We observed inverse associations between cortical bone area at the mid-tibia and FT3, TT3, TT4, and TBG. No associations between TSH and DXA or pQCT measurements were found. Conclusion: In healthy men at the age of PBM, between-subject variation in thyroid hormone concentrations affects bone density, with higher levels of FT3, TT3, TT4, and TBG being associated with less favorable bone density and content.