Oral calcium load test: diagnostic and physiologic implications in hyperparathyroidism

An oral calcium load test (CLT) (1 gm Ca/50 kg) was administered to 11 control subjects and 35 patients with overt hyperparathyroidism to assess its efficacy in diagnosis of hyperparathyroidism. All participants were placed on a low-calcium diet 3 days before the CLT. Intact parathormone and ionized calcium (Cai) levels were measured 0, 1, 2, and 3 hours after CLT. Initial Cai and parathormone (mean +/- SE) were 1.22 +/- 0.01 mmol/L and 2.94 +/- 0.03 pmol/L in the control group compared with 1.43 +/- 0.02 mmol/L and 10.6 +/- 2.2 pmol/L in the group with hyperparathyroidism. Both groups had a similar percent increase in Cai values (control, 5.9% +/- 0.8%; hyperparathyroidism, 6.3% +/- 0.6% (p greater than 0.1). A decline in parathormone levels of 47.6% +/- 2.8% in patients with hyperparathyroidism was significantly less than the 75.3% +/- 5.3% decline observed in control subjects (p less than 0.025). Three hours after CLT, parathormone was suppressed in control subjects, whereas a rebound occurred in patients with hyperparathyroidism. Postoperative CLT demonstrated a higher mean percent Cai increase and percent parathormone decline (Cai, 8.9% +/- 1.1%; parathormone, 67.9% +/- 1.8%) compared with preoperative values (Cai, 6.0% +/- 1.0%; PTH, 49.6% +/- 4.3%) (p less than 0.025), and 3 hours after calcium intake, parathormone remained suppressed, similar to control subjects. After surgery, three patients had elevated parathormone and low normal Cai levels and parathormone response to a CLT confirmed the diagnosis of secondary hyperparathyroidism. In conclusion, a CLT (1) can confirm the diagnosis of hyperparathyroidism and successful parathyroidectomy, (2) distinguished postoperative secondary from persistent primary hyperparathyroidism, (3) demonstrated nonautonomy of abnormal parathyroid glands with a parathormone response to a calcium load characterized by an earlier nadir, decreased suppressibility, and more rapid recovery, and (4) produced dynamic changes that did not distinguish patients with hyperparathyroidism from control subjects or hyperplasia from adenoma.