Parathyroid hormone radioimmunoassay (RIA), specific for mid-region of the PTH molecule, has been proven to be extremely useful for the differential diagnosis of abnormal calcium metabolism.
Recently, we developed a highly sensitive RIA for PTH, consisting of PTH antiserum (CH9), 125I labelled Tyr42 hPTH (43-68) and synthetic hPTH (1-84) as standard. This MA cross-reacted with mid-region and carboxyl terminals of PTH. The within-assay and betweenassay coefficients of variation were less than 4.6% and less than 8.6%, respectively. The limit of detection was 50pg/ml. The levels of serum calcium, serum phosphate, serum creatinine, Tmpo4/GFR and creatinine clearance (Ccr) in normal healthy volunteers aged 20 to 50 years remained almost constant and showed 9.24 ± 0.34mg/dl (mean ± SD, n = 242), 3.34 ± 0.38mg/dl (n = 242), 0.870 ± 0.121mg/dl (n= 242), 3.20 ± 0.54mg/dl GF (n=189) and 103 ± 17ml/min (n = 137), respectively. All healthy volunteers (n = 326) had measurements of PTH in the blood. From 20 to 50 years, normal PTH mean was 374 ± 97pg/ml (± SD, n= 237) and ranged from 180-568pg/ml, and from 60 to 80 years it was 471± 133pg/ml (n = 34) and ranged from 205-737pg/ml.
Since we found that PTH was markedly elevated above normal when Ccr was below 40ml/min, and PTH was very significantly correlated with the reciprocal of Ccr (r = 0.8996, P<0.001) using a multivariate analysis, all of the patients whose Ccr was higher than 40ml/ min were selected and examined in the following studies. Serum PTH values completely separated patients with surgically proven primary hyperparathyroidism (1°HPT) from malignant associated hypercalcemia (MAH), and patients wih idiopathic hypoparathyroidism (IHP) from pseudohypoparathyroidism (PHP), both of which were diagnosed by Ellsworth-Howard test. PTH values in all of the patients with 1°HPT (n = 23) were above normal, but those with MAH (n=6) were below the normal or lower normal range. PTH values in patients with PHP (n = 7) showed above normal, while those with IHP (n = 5) were below the normal range. PTH was normalized in post operative status in all patients after parathyroidectomy (n = 6).
These results indicate that this PTH RIA is extremely useful for the differential diagnosis in diseases with calcium abnormalities.
