Also Known As: IGFBP-3 Approximately 95% of the IGF-I and IGF-II are bound to IGFBP-3, which makes this protein the major carrier of IGFs in plasma. One of the principal functions of binding proteins is to extend the half-life of the IGFs from eight minutes to several hours. In this way, IGFBP-3 acts as a stabilizer, providing a constant level of IGF. The serum level of IGFBP-3 appears to be a constant over 24 hours and the protein was found to be growth hormone (GH) dependent, which makes the detection of IGFBP-3 very useful in the evaluation of growth hormone secretion. Single IGFBP-3 measurements correlate significantly with the logarithm of the integrated spontaneous GH secretions. Advantages of IGFBP-3 over IGF-1 are the higher levels in young children which gives a better discrimination between normal and subnormal levels. The IGFBP-3 assay is also used to diagnose children with short stature having normal GH levels. Some children with short stature give subnormal IGF-I levels despite normal GH levels. IGFBP-3 was found to be a more reliable parameter for monitoring the efficacy of therapy; therefore, it is a useful adjunct to the measurement of IGF-I.