Although serum
insulin-like growth factor I (
IGF-I) concentrations have utility as a screening test for
growth hormone (GH) deficiency in children and young adults, they are less accurate for screening in adults over 40 years of age. There are two main limitations in the clinical use of
IGF-I levels as a marker of GH secretion. First,
IGF-I synthesis is not only regulated by GH but also by nutrient supply and by other
hormones; second, low
IGF-I levels in the presence of normal or increased GH secretion may reflect a peripheral resistance to GH action. Although serum
IGF-I cannot be used as a stand-alone test for the diagnosis of adult GH deficiency, very low
IGF-I levels in the context of documented hypothalamic or
pituitary disease may be helpful in identifying patients with a high probability of GH deficiency. In the presence of two or more additional pituitary
hormone deficiencies, an
IGF-I level <84 microg/l (assayed by Esoterix Endocrinology, Inc. Calabasas Hills, CA, USA) indicates a 99% probability of GH deficiency. As this cut-off value has not been validated for other
IGF-I assays, an
IGF-I standard deviation score (SDS) of <-3 may be considered in adults over age 28; an even lower
IGF-I SDS is needed for diagnosis in younger adults. In clinical practice, other causes of low serum
IGF-I such as
malnutrition, diabetes,
hypothyroidism,
liver disease, etc., should be excluded before applying these diagnostic criteria.