Complex sentence: These sex hormones circulate in the blood stream mostly bound to SHBG, to some degree bound to albumin, and only a small fraction is unbound, or "free", and thus biologically active and able to enter a cell and activate its receptor.
Split into simpler sentences: These sex hormones circulate in the bloodstream, bound mostly to SHBG and to some degree bound to albumin. Only a small fraction is unbound, or "free," and thus biologically active and able to enter a cell and activate its receptor.