We report a method for carrier concentration tuning in the thermoelectric thiospinel Cu2CoTi3S8, which exhibits an n-type metallic character and a high power factor. An oxidative Cu extraction treatment produced Cu defects, resulting in Cu2-xCoTi3S8 up to x = 0.62. The electron carrier concentration was effectively reduced by this treatment, leading to the decrease in power factor, whereas the electronic contribution to the thermal conductivity was suppressed. As a result, the dimensionless figure of merit ZT remained unchanged as 0.2 at 670 K in the whole range of x <= 0.62. The oxidative Cu extraction described in this paper offers an opportunity to tune the electron carrier concentration for Cu-containing thermoelectric materials.