How do we best measure growth hormone action?

Growth hormone (GH) is a powerful anabolic hormone with a broad spectrum of action that has been assessed with three general parameters: auxological to assess the growth response; biochemical to measure anabolic effects; and body composition. In childhood, linear growth response is assessed with height, short-term changes in height velocity (HV), and attainment of final adult height, which may not be concordant. In both children and adults, the biochemical indices utilized to predict and/or monitor response to GH therapy have included: (1) nonspecific indices: glucose, insulin, urea, protein synthesis, lipid metabolism, and lipoproteins; (2) more specific indices of the GH-IGF axis: GH binding protein, IGF-I, IGFBP-3, and acid-labile subunit; or (3) indices of bone and mineral metabolism: calcium, phosphate, bone alkaline phosphatase, osteocalcin, propeptides of procollagen type I and III, and bone mineral content. For body composition, body mass index, total body % fat, total body or extracellular water, and bone mineral density have been addressed most frequently. Modest changes with wide variability have been observed with most measurements. GH dose is a very significant positive factor for all parameters. Few of the currently available tests can reliably predict and/or monitor response to GH therapy. Of these, serum IGF-I appears to offer the best integrated indicator of the action of GH throughout all age groups.