Bone Changes in Pre- and Postmenopausal Women with Thyroid Cancer on Levothyroxine Therapy: Evolution of Axial and Appendicular Bone Mass

The effects of suppressive doses of levothyroxine (LT₄) on bone mass are controversial. Our aim was to evaluate the effects on axial and appendicular bone mineral density (BMD) and bone metabolism of long-term LT₄ suppressive therapy in women by means of cross-sectional and longitudinal studies, and also to assess the potential influence of menopausal status and LT₄ dose. Seventy-six women (aged 47 + 13 years, 37 pre- and 39 postmenopausal) on suppressive therapy (67 + 34 months duration, mean LT₄ dose 168 + 41 mg/day) from our Thyroid Cancer Unit without previous hyperthyroidism or concomitant hypoparathyroidism were studied. Serum TSH, T₃ free T₄, calcium, phosphorus, alkaline phosphatase, BGP, iPTH and urinary calcium (uCA) were measured. BMD was measured by dual-energy X-ray absorptiometry (DXA) at lumbar spine, femoral neck, Ward's triangle, ultradistal and distal third radius and expressed as a Z-score. In a subset of 27 women aged 46 + 15 years (14 pre- and 13 postmenopausal) a second densitometry scan was performed 27 + 5 months later. Patients on suppressive therapy showed a small reduction in BMD at the distal third radius (Z-score: 70.77 + 0.98; 95% confidence interval: 71.11, 70.44) without differences between pre- and postmenopausal women. Significant relations with the regimen of suppressive therapy and bone turnover markers were detected except at the lumbar spine. In the longitudinal study a significant although mild reduction in femoral neck BMD was found that correlated with prior T₃ and iPTH. In conclusion, our data show a small detrimental effect of cautious LT₄ suppressive therapy on bone mass assessed by DXA; it remains to be established whether this increases the prevalence of fractures. Received: 30 June 1997 / Accepted: 7 November 1997