Effect of obesity and morbid obesity on the growth hormone (GH) secretion elicited by the combined GHRH + GHRP-6 test.

OBJECTIVE: Obesity is characterized by low basal levels of growth hormone (GH) and impeded GH release. However, the main problem arises in the diagnosis of GH deficiency in adults, as all accepted cut-offs in the diagnostic tests of GH reserve are no longer valid in obese subjects. In this work, the role of obesity in the GH response elicited by the GHRH + GHRP-6 test was assessed in a large population of obese and nonobese subjects. PATIENTS: GHRH + GHRP-6-induced GH peaks were evaluated in 542 subjects. One hundred and five were healthy obese, 50 were morbid obese, and 261 were nonobese (both normal weight and overweight). One hundred and seventy-six GH-deficient patients (obese and nonobese) were also studied.
RESULTS: A regression analysis of the 366 subjects with normal pituitary function indicated that adiposity had a negative effect on the elicited GH peak (r = -0.503, P < 0.0001). A receiver operating characteristic (ROC) curve analysis showed that in subjects with a BMI < or =35, the currently accepted cut-offs of the GHRH + GHRP-6 test (GH peaks > or =20 microg/l: normal secretion; GH peaks < or =10 microg/l: GH deficiency), were fully operative. However, in subjects with a BMI > 35, normality was indicated by GH peaks > or =15 microg/l and GH deficiency by peaks < or =5 microg/l (1 microg/l = 2.6 mU/l).
CONCLUSIONS: This study confirms: (a) that the combined provocative test is adequate to separate normal and GH-deficient subjects; (b) the negative effect of obesity on GH secretion; (c) that obesity accounts for 25% of the reduction of GH release; and (d) that present cut-off values are applicable to normal weight, overweight and grade I obesity subjects, whereas in obese subjects with a BMI exceeding 35, all the normative limits of the GHRH-GHRP +6 test must be reduced by 5 microg/l.