Fig. 1.

The decline in the human linear growth rate is not due to declining circulating IGF-I levels. First row, In humans, height increases rapidly in early childhood but eventually plateaus in adolescence (216). Second row, The linear growth velocity (first derivative of the height curve) decreases dramatically during infancy, more gradually during childhood, briefly rises during the pubertal growth spurt, and then resumes its decline, approaching zero (216). Third to fifth rows, As growth is slowing, there is a general increase in total IGF-I and IGFBP-3, both of which are stimulated by GH, as well as an increase in free IGF-I (derived from reference 52).