A catalytic regime for tritium in deuterium-tritium (DTx) fusion is investigated, including ion-electron collisions, mechanical expansion, bremsstrahlung radiation and inverse Compton scattering losses. It is shown that the consideration of the three main nuclear reactions of the hydrogen isotopes (D(D,n)3He, D(D,p)T and T(D,n)4He) only leads to erroneous results for internal tritium breeding. Among the secondary reactions in DT fusion, the reaction 3He(D,p)4He is the most important in the tritium catalytic regime, due to its large contribution to the plasma heating. When all the nuclear reactions are taken into account, internal tritium breeding is found to be possible in a short range of parameters defining the target performance (tritium content parameter x, areal density and ignition temperature).