It has been proposed that TSH has thyroid hormone-independent effects on bone mineral density (BMD) and bone metabolism. This concept is still controversial and has not been studied in human subjects in detail. We addressed this question by studying relationships between serum TSH concentration and indicators of bone turnover, after controlling for triiodothyronine (T₃), free thyroxine (FT₄), and non-thyroid factors relevant to BMD and bone metabolism. We also studied the contribution of the TSH receptor (TSHR)-Asp727Glu polymorphism to these relationships.

We performed a cross-sectional study with 148 patients, who had been thyroidectomized for differentiated thyroid carcinoma.

We measured BMD of the femoral neck and lumbar spine. FT₄, T₃, TSH, bone-specific alkaline phosphatase, procollagen type 1 aminoterminal propeptide levels, C-cross-linking terminal telopeptide of type I collagen, and urinary N-telopeptide of collagen cross-links were measured. Genotypes of the TSHR-Asp727Glu polymorphism were determined by Taqman assay.

We found a significant, inverse correlation between serum TSH levels and indicators of bone turnover, which was independent of serum FT₄ and T₃ levels as well as other parameters influencing bone metabolism. We found that carriers of the TSHR-Asp727Glu polymorphism had an 8.1% higher femoral neck BMD, which was, however, no longer significant after adjusting for body mass index.

We conclude that in this group of patients, serum TSH was related to indicators of bone remodeling independently of thyroid hormone levels. This may point to a functional role of the TSHR in bone in humans. Further research into this mechanism needs to be performed.