Osteoporosis: A bone turnover defect resulting from an elevated parathyroid hormone concentration within the bone-marrow cavity?

Recently the bone-marrow cavity blood concentration of parathyroid hormone (PTH) has been shown to exceed that of the peripheral blood. As PTH is a primary modulator of bone cell activity, altered levels of the hormone in the bone-marrow blood may play a significant role in the aetiology of bone disease. We therefore measured PTH concentrations in marrow cavity and venous blood of 9 osteoporotic and 14 control subjects using sequence specific radioimmunoassays for intact and mid-carboxyl (Mid-C) regional human PTH (hPTH). Intact and Mid-C PTH levels were identical in the peripheral blood of control and osteoporotic subjects. Furthermore, bone-marrow cavity blood concentrations of Mid-C PTH, whilst universally higher than those found in peripheral blood, were also comparable in the osteoporotic and control subjects. The sole difference in the PTH composition of bone-marrow cavity blood from osteoporotic subjects was an increased concentration of intact PTH. The origins and consequences of elevated levels of intact PTH within the marrow cavity blood of osteoporotic subjects are discussed.