Complex sentence: Circulating calcidiol may then be converted into calcitriol, the biologically active form of vitamin D, either in the kidneys or by monocyte-macrophages in the immune system.
Split into simpler sentences: Circulating calcidiol may then be converted into calcitriol, the biologically active form of vitamin D, in the kidneys. In addition to the kidneys, calcitriol is also synthesized monocyte-macrophages in the immune system.