The aim of our study was to test in healthy men whether PTH both exogenous (as a single subcutaneous injection of teriparatide) and endogenous (induced by an acute hypocalcaemia), could change acutely (minutes to hours) the serum concentrations of mature collagen type I degradation fragments (beta CTX) (Rosenquist et al. 1998), the marker which is released into the circulation during osteoclastic resorption of bone. Here, PTH is linked to Hypocalcemia.