Finally, post hoc comparisons using the Bonferroni t-test correction indicated that female SSc patients had: (1) lower TRP concentrations than female HC (MD = −8264.430; p < 0.001; 95%CI [−12,028.183; −4500.678]), (2) higher IDO and TDO activity than female HC (MD = 0.041; p < 0.001; 95%CI [0.019; 0.062]), and (3) higher KYNA/TRP activity than female HC (MD = 0.001; p = 0.001; 95%CI [1.09 × 10−4; 4.29 × 10−4]). This evidence concerns the gene TDO2 and systemic sclerosis.