High cathepsin K levels in men with differentiated thyroid cancer on suppressive L-thyroxine therapy

Background: Thyroid hormone administration is associated with low bone density in some studies. The aim of the present study was to evaluate the influence L-thyroxine, in doses used to treat patients with a history of thyroid carcinoma, on serum cathepsin K and other markers of bone metabolism. Cathepsin K is thought to have a role in osteoclast mediated bone resorption. Methods: A group of male patients with differentiated thyroid cancer (DTC) on suppressive L-thyroxine therapy (DTC-group; n = 51; mean age 57 years; TSH < 0.1 mU/L) was selected as a model for hyperthyroidism. The results were compared to a group of healthy euthyroid men (control-group; n = 50; mean age 58 years; TSH 1.5 +/- 0.9 mU/L). Results: In the DTC-group the median value of cathepsin K was 6.9 pmol/L, in the control group 4.8 pmol/L ( p = 0.0052; highly significant [h.s.]). There was a significant negative correlation of cathepsin K with age ( r = -0.279, p = 0.028). The analysis of various bone associated parameters revealed an increase of serum crosslaps in the DTC-group versus euthyroid controls ( p = 0.03). A significant correlation could be found for cathepsin K and osteoprotegerin ( p = 0.002). Conclusion: Cathepsin K is increased by a suppressive L-thyroxine therapy and decreases with increasing age. The increased cathepsin K levels seen in DTC-patients on suppressive L-thyroxine therapy are likely to contribute to accelerated bone degradation in these patients.