Normal values of circulating insulin-like growth factor-I bioactivity in the healthy population: comparison with five widely used IGF-I immunoassays - Brugts, M P; Ranke, M B; Hofland, L J; van der Wansem, K; Weber, K; Frystyk, J; Lamberts, S W; Janssen, J A (J Clin Endocrinol Metab, 2008)