Plasma 1,25-dihydroxyvitamin D and parathyroid hormone levels in paretic dairy cows.

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Abstract
Parathyroid hormone, 1,25-dihydroxyvitamin D, Ca, and P were determined in the plasma of 12 cows during the period immediately prior to, during, and after parturition. Eight were aged cows (.gtoreq. 3rd lactation), and 4 considered as young cows (.ltoreq. 2nd lactation). Four aged cows developed severe hypocalcemia (4.95 .+-. 1.1 mg/100 ml plasma) accompanied by paresis, whereas the 4 nonparetic aged cows experienced only mild hypocalcemia (8.43 .+-. 0.23 mg/100 ml plasma) at parturition. The mean .+-. SE plasma Ca value for the young cows was 9.02 .+-. 0.17 mg/100 ml at parturition. Development of severe hypocalcemia in the paretic cows was associated with increased plasma parathyroid hormone, 1,25-dihydroxyvitamin D and a decrease in plasma F. Plasma Ca and P were negatively correlated (within cow) with the increase in parathyroid hormone (r [correlation coefficient] = -0.57, r = -0.38) and the increase in plasma 1,25-dihydroxyvitamin D (r = -0.58, r = -0.34). Parathyroid hormone was positively correlated with the increased 1,25-dihydroxyvitamin D levels (r = 0.31). The development of hypocalcemia at parturition in dairy cows was apparently not due to failure of adequate 1,25-dihydroxyvitamin D.