IGF-I levels in different conditions of low somatotrope secretion in adulthood: obesity in comparison with GH deficiency.

BACKGROUND

It is widely accepted that IGF-I synthesis and release depend on GH secretion as well as on the nutritional status and vary with age. Based on these premises, after the definition of normal IGF-I levels during lifespan, in a large population of normal subjects of both sexes, our aim was to verify IGF-I levels in large groups of adult patients with GH deficiency or obesity, a condition in which a reduced somatotrope secretion is well known.

METHODS

To this goal, IGF-I levels were assayed after acidethanol extraction, in 326 normal subjects (NS, 98 men and 228 women, age 20-80 yrs, BMI 17.9-26.1 kg/m2), 54 patients with GH deficiency (GHD), 24 men and 30 women, age 20-80 yrs, BMI 18.2-27.1 kg/m2), and 195 patients with obesity (OB, 33 men and 162 women, age 17-71 yrs, BMI 27.7-64.9 kg/m2). In NS, IGF-I levels were similar in both sexes and showed a progressive decrease with age. No correlation was present between IGF-I and BMI in NS. Median IGF-I levels and the 3rd centile in NS when considered per decade were: III) 230 and 108.6; IV) 220 and 129.8; V) 150.5 and 72.4; VI) 163.0 and 62.4; VII) 110 and 41.6; VIII) 82 and 24.7 microgram/l. In GHD, IGF-I levels were independent on sex and did not show reduction during lifespan. Mean IGF-I levels in GHD were lower than that in NS (64 +/- 5.9 vs 171.3 +/- 4.8 microgram/l, p < 0.01) and did not correlate with age or BMI. Analyzing individual IGF-I levels, in GHD, in the III and IV decade 21/24 patients had IGF-I levels lower than 3rd centile while, up to the VIII decade, only 10/30 had IGF-I levels below normal limits. In OB, IGF-I levels were independent on sex but, like in NS, showed a progressive decrease with age and were independently, negatively correlated with BMI but not with WHR. Analyzing individual IGF-I levels, in OB, IGF-I levels were below 3rd centile in 10/77 patients in the III and IV decade and in only 8/108 patients up to the VIII decade. Mean IGF-I levels in the whole OB population (179.6 +/- 5.9 microgram/l) were higher (p < 0.01) than those in GHD (64.5 +/- 5.9 microgram/l) while only in the IV decade IGF-I levels in OB group were lower (p < 0.02) than those in NS (184.7 +/- 12.6 microgram/l vs 224.0 +/- 9.2 microgram/l).

CONCLUSIONS

In conclusion, present data confirm that IGF-I levels depends on GH secretion as well as on nutritional status, being negatively and independently correlated with age and BMI. IGF-I assay is not a reliable test for the diagnosis of GH deficiency in adulthood though it gives good discrimination between GHD and normal subjects up to 40 yrs of age. In spite of low GH secretion, IGF-I levels are only slightly reduced in obesity, probably as consequence of hyperinsulinism.