Maintenance of serum calcium after total thyroparathyroidectomy

We have studied calcium regulation in 11 consecutive patients undergoing radical surgery for upper aerodigestive tract malignancy. Eight patients received postoperative parenteral nutrition including calcium (19 mmol/day) and tri-iodothyronine (30 micrograms/day) supplementation. Three patients received enteral nutrition with calcium (70 mmol/day), 1.25 dihydroxycholecalciferol (1 microgram) and thyroxine (150 micrograms/day) via a nasogastric tube. Mean (SEM) corrected calcium fell from 2.42 (0.013) to 2.03 (0.036) mmol/l after 24 h (P less than 0.01). Replacement therapy generally maintained the serum calcium above 2.0 mmol/l. However, values were associated with only one episode of tetany. Phosphate increased from 1.10 (0.05) to 1.79 (0.11) mmol/l, 7-9 days postoperatively (P less than 0.001). Tubular calcium reabsorption fell and urinary calcium excretion rose, consistent with loss of parathyroid hormone (PTH) action on the distal nephron. However, the renal leak of calcium can be considerably reduced by concomitant salt depletion. This enhances proximal tubular sodium and calcium reabsorption thereby limiting calcium delivery to the distal nephron. This offsets the consequences of the loss of PTH which normally regulates distal calcium reabsorption.