Previous studies of shear-thinning fluids in pipe flow discovered that, although the time-averaged velocity profile was axisymmetric when the flow was laminar or fully turbulent, contrary to expectations it was asymmetric in the laminar-turbulent transition regime. We reveal that the asymmetry is not induced by the transition process, but is an instability of the laminar base state. Furthermore, the transition process is responsible for returning symmetry to the flow, which explains why the fully turbulent case is axisymmetric. The deviation from the axisymmetric laminar state is observed to develop in the form of a supercritical bifurcation with square-root dependence on Reynolds number. The asymmetry is non-hysteretic and reversible, not only having a favoured location, but a preferred route between axisymmetry and asymmetry. We also observed the greater the shear-thinning ability, the stronger the fluid asymmetry will

Keywords: Wall-bounded flows, Instability, Experimental techniques