Symptomatic hypocalcemia sometimes follows subtotal thyroidectomy for Graves' disease. Irreversible damage to the parathyroids contributes to permanent hypocalcemia and the mechanism for a transient hypocalcemia is thought to be different from that of a permanent one. However, sensitive assays for parathyroid hormones (PTH), which had recently become available, revealed that levels of PTH decrease in patients with transient hypocalcemia. In order to differentiate a prolonged hypocalcemia from a transient one, calcium and inorganic phosphate concentrations in serum as well as in urine, and whole molecule-PTH levels were determined in 18 Graves' disease patients with postoperative hypocalcemia just after the initial symptoms for hypocalcemia appeared. In 13 patients, medication was withdrawn within one month since serum calcium levels had returned to normal (transient hypocalcemia). In five other patients, medication was required for six months or more to maintain normocalcemia (prolonged hypocalcemia). The same parameters were determined after surgery in eight Graves' disease patients without hypocalcemia. Urinary inorganic phosphate concentrations in patients with prolonged hypocalcemia (0.02+/-0.01 mmol/mmol Cr) were significantly lower (P<0.01) than those in patients with transient h, pocalcemia (1.59+/-1.59 mmol/mmol Cr) or those in control patients (1.27+/-0.70 mmol/mmol Cr). Preoperative concentrations of calcium and inorganic phosphate in serum and urine, and serum alkaline-phosphatase activities were also determined. However, there were no significant differences in these parameters between patients with prolonged and those with transient hypocalcemia. It is concluded that prolonged hypocalcemia is discriminated from the transient type by determining the urinary inorganic phosphate at the time of appearance of the initial symptoms for hypocalcemia. A decrease in urinary inorganic phosphate concentrations, which are easily determined, indicates accurately that hypocalcemia will be prolonged.
