Evaluation of hypocalcemia with a highly sensitive, homologous radioimmunoassay for the midregion of parathyroid hormone.

The diagnosis of hypoparathyroidism by radioimmunoassay of serum parathyroid hormone (PTH) has been hampered by lack of an assay system sensitive enough to allow discrimination between low and normal values. A new assay for human PTH that has improved sensitivity has been developed. It uses an homologous antiserum (against the human hormone) and uses a carboxy-terminal fragment of bovine PTH as tracer to provide an assay monospecific for the midregion of PTH. Immunoreactive PTH (iPTH) was detectable in 25/27 normal children and borderline detectable in the other two. The pediatric normal range was slightly lower than that previously established in adults. Among patients with secondary hyperparathyroidism and normal renal function, iPTH was 3- to 8-fold elevated in those with rickets, and 1.3- to 2.0-fold above normal in those with more acute forms of hypocalcemia. Twelve patients with hypoparathyroidism were studied. iPTH was undetectable in seven with permanent total hypoparathyroidism, and was borderline detectable in five, including four neonates who proved to have transient hypoparathyroidism. In these four patients, iPTH became detectable as the requirements for supplemental calcium decreased. Measurement of iPTH with an adequately sensitive assay may be useful in the diagnosis and management of pediatric hypocalcemia.