Insulin‐like growth factor‐I in growth hormone‐deficient adults: relationship to population‐based normal values, body composition and insulin tolerance test

BACKGROUND: Although an insulin tolerance test (ITT) is the most commonly used method for detecting growth hormone (GH) deficiency (GHD) in adults, measurements of serum insulin-like growth factor-I (IGF-I) may also be of value. OBJECTIVE: To validate the use of serum IGF-I concentration in the diagnosis of GHD in adults. DESIGN: A cross-sectional study. PATIENTS: One hundred and four patients, 60 men and 44 women, with known pituitary disease and verified GHD based on ITT. MEASUREMENTS: Serum IGF-I was determined by radioimmunoassay after acid-ethanol extraction. Body composition was estimated with total body potassium combined with total body water assessments. RESULTS: According to age- and sex-adjusted population-based references values, 51 patients had serum IGF-I concentrations below -2 SD of the predicted values and 53 had concentrations within 2 SD. Fifty-seven per cent of the patients aged 41 years (25th percentile) or below and 39% of the patients aged 57 years (75th percentile) or above had serum IGF-I concentrations below -2 SD. Women had lower mean IGF-I SD scores than men (P < 0.01). Serum IGF-I was correlated with peak GH response during ITT (r = 0.40; P < 0.001), age (r = -0.27; P < 0.01), duration of hypopituitarism (r = -0.52; P < 0.001), number of pituitary hormonal deficiencies (r = -0.35; P < 0.001), body cell mass (r = 0.30; P < 0.01) and serum insulin (r = 0.21; P < 0.05). The peak GH response during ITT correlated with spontaneous GH secretion, duration (P = -0.48; P < 0.001) and number of deficiencies (r = -0.50; P 0.001). CONCLUSION: The measurement of serum IGF-I concentrations is not suitable as a single diagnostic test for growth hormone deficiency in adults. Even as a screening test, its use appears to be limited, especially in elderly subjects. The serum level of IGF-I was influenced by several factors in addition to GH, such as age, gender, anthropodometry and serum insulin level. The peak GH response during the insulin tolerance test appears to be influenced to a lesser degree by these factors.