Adolescents also undergo a pubertal growth spurt, when they grow faster than at any other time of life, aside from fetal and early infant growth. Rising growth hormones is directly (and indirectly through estrogen and testosterone) responsible for the growth spurt. Another hormone involved in the pubertal growth spurt is insulin-like growth factor I (IGF-I), which rises very ealry. Unlike levels of growth hormone, initial rises in IGF-I levels are independent of gonadal hormones