Bone Abstracts

In previous studies, discrepant effects of TSH and thyroid hormones on bone have been reported. While low TSH and high thyroxine (T₄) levels increase bone turnover, a recent study suggested a positive association between triiodothyronine (T₃) levels and mineral deposition in bone. No data are available on bone architecture and strength in relation to thyroid status.

Aim: Using a cross-sectional design, we determined whether bone density, structure, and strength are affected in patients with post-surgical hypothyroidism on T₄ substitution therapy.

Patients and methods: We compared 45 patients with well-substituted hypothyroidism with 45 age- and sex-matched controls. Areal BMD was assessed by DXA. Volumetric BMD and bone geometry was measured by HR-pQCT scans (distal radius and tibia). Bone strength was estimated by finite element analysis.

Results: Mean age was 55 years. 80% were women. Patients had been on treatment with T₄ for 11.5 (range 3–41) years. Patients and controls had similar levels of TSH. Patients had significant higher T₄- but lower T₃-levels. Areal BMD did not differ between groups at the lumbar spine, hip, forearm or the whole body. Neither did cortical or trabecular volumetric BMD or indices of bone geometry differ between groups. Finite element analysis showed no significant difference in failure loads. For the entire population, there was no significant correlation between BMD and TSH or T₄, but T₃ was significant positively correlated to BMD at the hip (P=0.324, P<0.01), spine (P=0.225, P<0.05), and the distal forearm (P=0.278, P<0.05). Furthermore, T₃ correlate positively to failure load at radius (P=0.386, P<0.01) and tibia (P=0.302, P<0.01).

Conclusion: If patients with hypothyroidism are well-substituted with T₄, the disease does not affect bone to any major degree. Our findings of an increased BMD and improved bone strength with increased T₃ levels call for further studies on potential beneficial effects of T₃ on bone metabolism.