The plasma concentrations of parathyroid hormone (PTH), ionised calcium (Ca2+), total calcium, albumin and inorganic phosphorus, and the pH were measured in blood samples obtained from nine dogs during a period of 26 hours. The plasma PTH levels fluctuated slightly during the day, by about 20 pg/ml, but there was a distinct peak (42·8 [8·8] pg/ml) at 07.00. Plasma Ca2+ showed a diurnal pattern in which two peaks (increases of 0·03 mmol/l) were observed at 05.00 and 17.00, and the plasma concentration of inorganic phosphorus showed a similar pattern. There were no diurnal changes in total calcium or albumin.
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