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Comparison of Urinary Growth Hormone and IGF-I Excretion in Small- and Appropriate-for-Gestational-Age Infants and Healthy Children

ABSTRACT: The output of urinary growth hormone (GH) and IGF-I were quantitated by RIA in 12-h urine collections obtained from infants who were preterm, small for gestational age (PT-SGA, n = 13); preterm, appropriate for gestational age (PT-AGA, n = 27); full term, small for gestational age (FT-SGA, n = 13); and full term, appropriate for gestational age (FT-AGA, n = 29); and from normal children (n = 33). The amounts of GH and IGF-I (mean ± SEM) excreted by the PT-SGA and FT-SGA infants were not significantly lower than those excreted by the PT-AGA and FT-AGA groups, respectively [GH (μg/kg): PT-SGA 13.7 ± 3.1 versus PT-AGA 14.0 ± 2.2, FT-SGA 7.8 ± 2.4 versus FT-AGA 6.6 ± 1.8; IGF-I (nmol/kg): PT-SGA 0.52 ± 0.09 versus PT-AGA 0.53 ± 0.04, FT-SGA 0.31 ± 0.05 versus FT-AGA 0.35 ± 0.04]. All infant groups exhibited significantly greater outputs of urinary GH and IGF-I compared with the children (p < 0.01). The plasma concentrations of GH in all infant groups were high, whereas the plasma IGF-I levels were low. Microalbumin and β-2 microglobulin excretion did not correlate with urinary GH and IGF-I output. Despite the higher microalbumin output in FT babies, urinary GH and IGF-I excretion was lower in these groups. The high plasma and urinary GH levels in the presence of increased urinary IGF-I output suggest that production of these peptides in early life is increased and they may reflect the hormonal mechanisms controlling the rapid somatic growth observed in early infancy. However, we cannot rule out the possibility that urinary IGF-I output is a reflection of renal IGF-I synthesis rather than generalized IGF-I production. Based on the comparison of urinary GH and IGF-I excretion in SGA and AGA infants, it appears that postnatal production of these peptides in SGA infants is not impaired.