Circulating free insulin-like growth-factor-I (IGF-I) levels should also be measured to estimate the IGF-I bioactivity.
Free IGF-I by analogy with sex and adrenal steroids and thyroid hormones, may be the major biologically active hormonal form of IGF-I. Because of methodological difficulties in measuring the free IGF-I the measurement of total IGF-I in blood is often used to assess the activity of the endocrine GH-IGF-I axis in clinical studies. However, there is currently no reliable standard reference method for circulating total IGF-I against which individual samples can be calibrated. In addition, in many of the common methods used to measure circulating total IGF-I levels, remaining insulin-like growth factor, binding proteins (IGFBPs) or binding protein fragments after sample extraction, may still interfere and produce falsely increased or decreased circulating total IGF-I levels. This latter phenomenon occurs especially under pathologic conditions. In addition, it has also been suggested that altered post-sampling integrity of IGF-I in vitro might contribute to the reported inconsistencies in circulating total IGF-I levels in literature. Although at the moment there is also no "golden standard" for the measurement of circulating free IGF-I levels, we discuss some studies in this paper that in our opinion, have demonstrated conclusively that circulating free IGF-I levels in several conditions reflect the IGF-I bioactivity better than circulating total IGF-I levels. Therefore, when evaluating the IGF-I bioactivity in health and disease, we recommend measuring also circulating free IGF-I.
Janssen
J A M J L
JA
Department of Internal Medicine, Erasmus MC, Rotterdam, The Netherlands. j.a.m.j.l.janssen@erasmusmc.nl
van der Lely
A J
AJ
Lamberts
S W J
SW
D001769:Blood / Q000378:metabolism; D004704:Endocrinology / Q000379:methods; D019382:Human Growth Hormone / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D012016:Reference Values
D007328:Insulin; D019382:Human Growth Hormone; D007334:Insulin-Like Growth Factor I