Oxytocin, the molecule often called “the bonding hormone” for its role in trust, attachment and reading other people’s emotions, also seems to have something to say about gender identity. A team of Iranian researchers has just published the first study to observe elevated levels of this substance in transgender men, and a notably high activity of its receptor gene in those undergoing testosterone treatment.
The study, published in Scientific Reports, tackles a question that remains open: what happens at a biological level in gender dysphoria, the sense that the sex assigned at birth does not match one’s lived identity. Oxytocin acts on brain circuits involved in the social and the emotional, and its effects pass through the receptors carried by its target cells. That its levels vary is striking, because it connects the physical with the most intimate emotional experience.
What the researchers measured
Fifty-nine people took part: 18 trans men who had not started hormonal treatment, 11 trans men who were injecting testosterone, and 30 cisgender women as a comparison group. In all of them, blood tests measured the levels of several hormones, including oxytocin, estradiol, progesterone and free testosterone. A genetic analysis technique then measured how much the oxytocin receptor gene (OXTR) was being expressed in the blood.
The main finding was twofold. On one hand, plasma oxytocin concentrations were significantly higher in trans men overall than in cisgender women. On the other, expression of the OXTR receptor gene was clearly increased in the group using testosterone, compared with the control group.
Changes pointing to a mechanism
The authors stress that this does not mean oxytocin “causes” dysphoria; rather, the finding opens a path toward understanding its mechanisms. The relationship could run in both directions: identity and the emotional environment can influence oxytocin, which in turn modulates how we process relationships and emotions. Nor is it clear whether the observed increase contributes to the symptomatology or is a consequence of it — a question that only future studies can answer.
One especially interesting observation concerns the receptor gene. That its expression is higher precisely in the group receiving testosterone suggests that gender-affirming hormonal treatment could be modulating oxytocin biology, an effect not described until now.
Why it matters
Gender dysphoria has a neurobiological and genetic basis that is still poorly understood. Studies like this one, though small and preliminary, help move the debate from the merely psychological or social toward a fuller understanding of the phenomenon. Measuring oxytocin and its receptor opens a door to biological markers that could one day guide clinical care.
As the researchers themselves warn, more studies are needed to confirm whether these changes are cause or consequence, and with larger samples. For now, the finding stands as one more piece of the puzzle: the bonding hormone also seems to speak of identity.
Source: Dastorani M, Shahini N, Livani A, Saghaeian Jazi M, Asadi J. Elevated oxytocin levels and oxytocin receptor gene expression in female-to-male transgender individuals. Scientific Reports. 2025;15:42729. doi:10.1038/s41598-025-26845-w.
