Plays an important role in growth control. Its major role in stimulating body growth is to stimulate the liver and other tissues to secrete IGF-1. It stimulates both the differentiation and proliferation of myoblasts. It also stimulates amino acid uptake and protein synthesis in muscle and other tissues. Monomer, dimer, trimer, tetramer and pentamer, disulfide-linked or non-covalently associated, in homomeric and heteromeric combinations. Can also form a complex either with GHBP or with the alpha2-macroglobulin complex. Expressed in the placenta. Belongs to the somatotropin/prolactin family.