In contrast to the elevated TRAP serum levels with hyperthyroidism, in vitro treatment of premature osteoclasts with T3 until their maturation did not affect the expression of acid phosphatase 5, tartrate resistant (Acp5) (Fig. 1n), the gene encoding for TRAP, indicating that actions of thyroid hormones on osteoclasts might be indirectly mediated via other bone cells. This evidence concerns the gene ACP5 and hyperthyroidism.