Growing evidence suggests that striatal-related behaviors and pathologies display clear sexual dimorphism. Previous research linking sex and striatal function has focused primarily on the effects of spiny projection neurons (SPNs). Sex hormones have been proposed to also modulate the activity of striatal interneurons, although this modulation has only been investigated at the behavioral level. In this study, we combined ex vivo pharmacological and optogenetic approaches with fiber photometry to examine how sex and the estrous cycle affect the FSIs’ integration of thalamic inputs and intrinsic excitability. We found that FSIs responsiveness to thalamic inputs and active membrane properties were influenced by both sex and estrous cycle. By pharmacologically manipulating estrogen-mediated signaling, we were able to exacerbate the excitability differences observed in FSIs, indicating that estradiol may play a role in modulating FSI ionic conductances. These findings identify modulation of the thalamus-FSI microcircuit as a potential substrate for sex differences in striatal-based behavior and disorders caused by striatal dysfunction.