We have used the local-density formalism and the atomic-sphere approximation to calculate self-consistently the electronic properties of thorium at pressures up to 400 kbar. The derived equation of state agrees very well with static pressure experiments and shock data. Below the Fermi level (EF) the electronic band structure is formed by 7s and 6d states while the bottom of a relatively broad 5f band is positioned 0.07 Ry above EF. The calculated extremal areas of the Fermi surface and their calculated pressure dependence agree with earlier calculations and with de Haas-van Alphen measurements supporting the validity of the itinerant description of the 5f electrons for the light actinides. The calculation shows that the gradual s to d transition taking place at pressures up to 200 kbar is the cause of the unusual pressure dependence of the Fermi surface seen experimentally.