Bone mineral density in Klinefelter syndrome is reduced and primarily determined by muscle strength and resorptive markers, but not directly by testosterone

Klinefelter syndrome (KS) patients have lower bone mineral density (BMD) at the spine, hip and forearm compared to healthy subjects, but frank osteoporosis is not common. Muscle strength and bone markers predicted BMD but KS itself and serum testosterone did not. Low vitamin D and high PTH were frequent among KS. The long-term consequence of KS on bone health is not well described. The objective of this study is to investigate the regional BMD and its determinants in KS. This is a cross-sectional study. BMD at the spine, hip and forearm are measured by DXA and correlated to biochemical markers of bone turnover, vitamin D metabolites, PTH, sex hormones, growth factors as well as muscle strength and anthropometric measures. The setting is at a university clinical research centre. The study involves 70 adult KS patients and 71 age-matched healthy subjects. In KS, BMD was universally lowered in all regions. Markers of bone formation or bone resorption were not altered in KS, but 25-OH-Dvitamin was lower (55 vs. 82 nmol/L, p < 0.0001) than in healthy subjects. Significantly more KS patients had low BMD (Z-scores below -2) at the forearm (15 KS vs. two healthy subjects, p = 0.001) but not at the spine or hip. Muscle strength (bicep and quadriceps) was lower among KS patients. Multivariate analysis revealed that muscle strength, treatment with testosterone (ever/never), age at diagnosis, SHBG, bone-specific alkaline phosphatase and 1CTP were all independent predictors of BMD, but androgens was not. KS patients had lower BMD at the spine, hip and forearm compared to age-matched healthy subjects, but frank osteoporosis was not common. Muscle strength, previous history of testosterone treatment, age at diagnosis and bone markers were predictors of BMD, but testosterone was not. Signs of secondary hyperparathyroidism were present among KS. Dietary intake of vitamin D or sun exposure may be lower in KS patients.