Calcium homeostasis during hypertonic volume expansion in the hypothyroid rat

The clearance characteristics of calcium and the relationship of coupled Na+ and Ca++ renal transport were evaluated in the hypothyroid rat (TX) during volume expansion with 1.5% saline. Despite similar hydropenic plasma calcium, after 11 ml/100 g body wt of infusion volume, the plasma calcium was significantly lower in the TX group (TX, 6.1 +/- 0.2; TI, 7.0 +/- 0.1; P less than .001). With continued infusion, plasma calcium in the TX group continued to drop to 5.3 +/- 0.3 while in the control rats (TI group) it rose to 8.0 +/- 0.1. This marked hypocalcemia in the TX group could not be explained by urinary losses of calcium, hemodilution, deficient parathyroid response, or changes in ultrafilterable plasma calcium. The higher fractional calcium excretion found in the TX group paralleled the higher fractional sodium excretion, and the coupling between calcium and sodium excretion was preserved in hypothyroidism under these conditions. Our results indicate an abnormal calcium homeostasis in the hypothyroid rat. The data do not support a renal cause for this disturbance but are consistent with the view that mobilization of calcium from the bone is sluggish in hypothyroidism during volume expansion.