Assessment of GH/IGF-I axis in obesity by evaluation of IGF-I levels and the GH response to GHRH plus arginine test

The GH response to provocative stimuli in obese is often as low as in panhy popituitaric patients with severe GHD; however, IGF-I levels are normal or slightly reduced. In 53 patients with simple obesity (11 M and 42 F, age: 4 0.3+/-1.6 yr, BMI: 39.1+/-1.0 Kg/m(2)), we evaluated the GH response to GHR H (1 mu g/kg iv)+arginine (ARG, 0.5 g/kg iv), and total IGF-I levels. The m ean (+/-SE) GH peak after GHRH+ARG was markedly lower (74% reduction, p<0.0 001) in obese (16.8+/-2.0 mu g/l) than in normal subjects (62.7+/-4.3 mu g/ l). IGF-I levels in obese patients (134.0+/-7.6 mu g/l) were lower (33% red uction, p<0.001) than in normal subjects (200.8+/-5.7 mu g/l). Taking into account the 3(rd) centile limit of normal response, the GH response to GHRH +ARG was reduced in 62.3% (33/53) of the obese patients, and 21.2% (7/33) o f them had low IGF-I levels. Assuming the 1(st) centile limit, it was reduc ed in 33.9% (18/53) obese subjects, and 22% (4/18) of them had low IGF-I le vels. Considering 3.0 mu g/L as arbitrary cut-off, the GH response was redu ced in 5.7% (3/53) of the obese patients, and still one of them had low IGF -I levels. Our findings: a) confirm that the secretory capacity of somatotr oph cells is often deeply impaired in obesity; b) demonstrate that IGF-I as say generally rules out severe impairment of GH/IGF-I axis in obese patient s with marked reduction of the GH secretion; c) indicate that the percentag e of obese patients with concomitant reduction of GH secretion and ICF-I le vels is not negligible. Thus, IGF-I assay should be routinely performed in obese patients; those presenting with low IGF-I levels should undergo furth er evaluation of their hypothalamo-pituitary function and morphology, parti cularly in the presence of empty sella. (C) 1999, Editrice Kurtis.