Markers of bone turnover, parathyroid hormone, calcium, and magnesium concentrations in horses with acute colitis

Hypocalcemia is frequent in horses with colitis. Information on serum parathyroid hormone (PTH) concentrations and its association with bone turnover biomarkers in horses with colitis is lacking. We aimed to determine the association between serum bone resorption biomarkers (C-terminal telopeptide of type I collagen [CTX-I]) and bone formation (osteocalcin [OCN]) with blood PTH, total calcium (tCa), ionized calcium (iCa), phosphorus (Pi), and total magnesium (tMg) concentrations, and mortality in horses with acute colitis. A total of 163 horses were divided into colitis (n = 127) and healthy (n = 36) groups. Blood samples were collected from all horses. Non-parametric methods were used for data analysis. In horses with colitis, serum CTX-I and PTH concentrations were significantly higher, whereas OCN, tCa, iCa, Pi, and tMg concentrations were lower compared to healthy horses (P < 0.05). In colitis horses with ionized hypocalcemia, serum CTX-I concentrations were positively correlated with PTH concentrations (P < 0.05). Colitis horses with iCa concentrations < 1.4 mmol/L and PTH > 92.9 pg/mL were more likely to die ([OR = 6.1; 95 % CI = 1.2-24.5; P < 0.05]; [OR = 3.6; 95 % CI = 1.3-10.1; P < 0.05]), respectively. In colitis horses, elevated PTH and CTX-I concentrations, together with decreased OCN concentrations, suggests that bone turnover activity was increased to offset hypocalcemia. Elevated PTH and hypocalcemia were associated with non-survival, indicating that in non-surviving horses, bone resorption activity was not sufficient to restore normocalcemia. PTH resistance could have contributed to hypocalcemia in some horses with acute colitis.