Growth hormone immunoreactivity does not reflect bioactivity

In childhood, the largest secretory burst of GH occurs during nighttime, an d consists of a complex mixture of molecular forms of GH that are thought t o have different biologic activity. Standard GH assays cannot distinguish b etween bioactive and biologically inactive GH isoforms. To examine this rel ationship, overnight GH secretion was assessed by blood sampling every 30 m in in 10 short prepubertal children (7 boys and 3 girls) to evaluate both t he serum concentration and the biologic activity of GH. Serum GH concentrat ions were measured by an immunofluorometric assay and its biologic activity by the Nb2 cell bioassay. The 12-h (2000 h to 0800 h) and 6-h (2000 h to 0 200 h and 0200 h to 0800 h) GH profiles were analyzed using the Pulsar prog ram. When GH secretory pattern was measured by immunofluorometric assay, th e area under the curve above the 0 line, the mean GH concentration, and the mean height of the secretory peaks were significantly higher during the fi rst than during the second part of the night (29.17 +/- 5.93 versus 16.29 /- 1.87 mIU/L, p < 0.05; 7.77 +/- 1.28 versus 4.83 +/- 0.33 mIU/L, p < 0.05 ; 4.61 +/- 0.94 versus 2.68 +/- 0.27 mIU/L, p < 0.05, respectively). In con trast, GH biologic activity was not significantly different during the two parts of the night. In conclusion, a dissociation between GH bioactivity an d immunoreactivity is present in physiologic conditions, indicating that st andard GH measurements do not provide any information on the biologic activ ity of the hormone. Although GH secretion is regulated by complex neuroendo crine mechanisms, the biologic activity of the hormone seems to be independ ent of them and is most probably regulated by peripheral mechanisms acting on its clearance or bioavailability to the target tissues.