Sodium Bicarbonate Infusion Test: A New Method for Evaluating Parathyroid Function

We have developed a new test for estimating the secretory capacity of parathyroid hormone (PTH) from the parathyroid gland. Sodium bicarbonate solution [8.4% (w/v); 35 ml/m2 body surface area] was infused for 2 min, and blood samples for the determination of plasma ionized calcium, plasma PTH (intact, midregion, carboxy-terminus) and related parameters were serially obtained. In 8 healthy volunteers, the mean (±SE) plasma ionized calcium fell promptly and significantly (from 1.21 ± 0.01 to 1.11 ± 0.01 mmol/L) after the sodium bicarbonate infusion. The mean (±SE) plasma intact PTH increased promptly and significantly, by more than four fold (42.3 ± 4.2 to 182.4 ± 34.7 pg/ml), and then gradually returned to basal levels. In patients with partial hypoparathyroidism who have detectable basal plasma levels of PTH, the absolute increment in PTH levels was much less, and in the plasma obtained from patients with complete hypoparathyroidism, absolutely no response was observed. Plasma obtained from patients diagnosed with primary hyperparathyroidism (parathyroid adenoma or hyperplasia) has high basal PTH levels. The response to the sodium bicarbonate infusion in these patients was markedly blunted (less than a two-fold increase in all cases examined). No significant adverse effects were observed during the procedure. Therefore, the sodium bicarbonate infusion test is a simple and sensitive method to stimulate PTH release, and is clinically useful for evaluating parathyroid gland function.