24-hour secretion of growth hormone (GH), insulin-like growth factors-I and -II (IGF-I, -II), prolactin (PRL) and thyrotropin (TSH) in young adults of normal and tall stature.

Abstract

Studies of the growth hormone (GH) secretory dynamics of children with normal and idiopathic short stature (ISS) have revealed that the regulation of the GH-somatomedin (GHS) axis can differ significantly among normal individuals. Information on the GH secretion in idiopathic tall stature (ITS) is scarce. We previously showed that the GH response to stimulation with GH-releasing hormone (GHRH) in male, late adolescents and young adults with ITS is significantly greater than that of their sex and age-matched controls of average height. In the present study, we studied the 24-hour (hr) GH, insulin-like growth factor-I and -II (IGF-I and -II), prolactin (PRL) and thyroid-stimulating hormone (TSH) secretion by every 30 minutes (min) sampling in 12 young, healthy male Greek army recruits. Group I [n=6, age 22+1.4 years (y.), mean + standard deviation (SD)] had a height of 198.5+4.2cm, at least 3 SD's above the mean of the Greek male population. Group II (n=6, age 20.5+1.05 y.) had a height of 169.2+3.4cm, within 2SD's of the normal mean. Serum IGF-I levels were determined in both unextracted and extracted samples. Our results indicated that the number of secretory bursts and the circadian, panel of GH, IGF-I and -II, PRL and TSH were similar in the two groups. Both the amplitude of the secretory GH peaks (5.08+3.07 vs. 3.3+0.8 ng/ml, p=0.19,) and the area under the curve (AUC) of the 24-hour GH secretion (9.8+5.5 vs. 6.6+1.3 ng/ml/hr, p=0.2) were higher in group I than in group II, but the difference was not significant. A significant nocturnal increase of both IGF-I and -II levels was found only in extracted human plasma (p<0.001), whereas measurements of IGF-I in unextracted samples failed to reveal circadian variation (p<0.1). We conclude that no significant differences were found in this pilot study of the neurosecretory regulation of the GHS axis between individuals of tall and normal stature. A tendency for greater amount of GH secretion per secretory peak was found in persons with tall stature; however, this finding needs to be confirmed in a larger study. IGF-I and -II levels had a significant circadian variation with a large nocturnal surge, when measured in extracted plasma. The latter, might be explained by circadian variation of the circulating IGF-binding proteins and its detection appears to be method of extraction-dependent.