Thyroid hormones play important roles in bone growth, development, and turnover. To exert its biological activity, thyroxine (T4) needs to be converted to 3,5,3'-triiodothyronine (T3) by iodothyronine deiodinase. In human thyroid gland as well as rat brown adipose tissue, type 2 iodothyronine deiodinase (D2) expression is regulated by a TSH (TSH) receptor-cAMP mediated mechanism. TSH receptor knockout mice demonstrated the direct effects of TSH on bone via TSH receptors found on osteoblast and osteoclast precursors. In the present study, we investigated the possible expression and function of iodothyronine deiodinase and TSH receptors in human osteoblast-like osteosarcoma (SaOS-2) cells and normal human osteoblast (NHOst) cells. Iodothyronine deiodinase activity was detected in SaOS-2 cells and NHOst cells, and all the characteristics of the deiodinating activity were compatible with D2. Northern analysis demonstrated D2 mRNA expression in SaOS-2 cells and NHOst cells. D2 mRNA levels as well as D2 activities were rapidly increased by (Bu)2cAMP or forskolin in SaOS-2 cells and NHOst cells. TSH receptor mRNA was demonstrated in SaOS-2 cells and NHOst cells, and D2 mRNA and D2 activity were stimulated by TSH in both cells. In addition, all the T3 receptor isoforms were detected by RT-PCR in SaOS-2 cells and NHOst cells. The present results indicate the expression of functional TSH receptors and D2 in human osteoblasts, and suggest previously unrecognized roles of TSH receptors and local T3 production by D2 in the pathophysiology of human osteoblasts.