The precise mechanisms involved with the increased REE observed in the present study cannot be identified clearly. However, considering the demonstrated toxic effects of PTH excess on various organs and body systems, some possibilities can be raised. Besides bone, several lines of evidence indicate that skeletal muscle is also a target organ for PTH. In fact, muscle dysfunction and wasting are common features of clinical states with PTH excess such as primary and secondary HPT(3-5). It seems that excessive PTH also affects bioenergetics of skeletal muscle, impairing energy production, transfer , and utilization(6). In addition, the hormone may enhance muscle proteolysis and increase release of alanine and glutamine in vitro(7). Indeed, negative nitrogen balance and net protein loss were observed in patients with primary HPT(18,19). PTH also play a role in the development of glucose intolerance by interfering with the ability of the pancreatic B cells to augment insulin secretion appropriately in response to the insulin-resistant state (20,21). Impaired peripheral glucose utilization and cellular energy supply may affect protein metabolism(20). Although we have not evaluated any parameter related to protein metabolism, the inverse correlation found between PTH and markers of muscle mass, such as LBM and MAMC, and that PTH was an independent determinant of LBM may suggest indirect evidence regarding the action of PTH in muscle (Figure 2, Table 4).