Gender dysphoria in monozygotic twins: a case report of two transgender male siblings receiving simultaneous cross-sex hormone therapy

Introduction

Gender dysphoria refers to the distress arising from the incongruence between an individual’s experienced gender identity and the sex assigned at birth. Genetic, biological, and psychosocial factors are all considered to contribute to its etiology. Monozygotic twin studies provide a useful framework for examining these influences, with reported concordance rates of 10–20%, a finding consistent with genetic susceptibility rather than strict determinism. Despite growing clinical interest, cases in which monozygotic twins initiate cross-sex hormone therapy simultaneously remain rarely documented.

Objectives

This report describes the clinical course of monozygotic twin siblings, both identifying as transgender male, who presented to psychiatry independently and subsequently initiated cross-sex hormone therapy concurrently. The aims were to compare clinical presentations across the two cases, to illustrate the value of an integrated psychiatric and endocrinological approach, and to contribute to the limited literature on gender dysphoria in twin populations.

Cases

Both individuals were 30-year-old transgender males, employed as couriers and living together in Istanbul. Each presented independently to psychiatry in October 2022.

The first case (MKA) reported a longstanding sense of being male, consistent avoidance of feminine clothing throughout childhood, and recalled cutting his own hair short at age seven. Breast development and the onset of menstruation at twelve were significant sources of distress during adolescence. Identifying as heterosexual, MKA reported marked discomfort with undressing during sexual encounters.

The second case (MEA) described body dysphoria that became more pronounced around puberty, alongside distress at being perceived as female. From childhood he had gravitated toward male peers, adopted a masculine style of dress, and worn his hair short since age ten. In a six-year relationship, MEA reported genital dysphoria during sexual activity and noted imagining himself in a male role during masturbation. Their mother, who had raised the twins alone, described a strict and structured upbringing.

Both cases underwent individual psychiatric interviews, family assessments, and endocrinological evaluation according to DSM-5-TR criteria. Hormone replacement therapy was initiated in February 2025 following approximately two and a half years of follow-up. Voice deepening and increased body hair were observed in both individuals and were associated with a meaningful reduction in dysphoric symptoms. MEA additionally reported relief following the cessation of menstruation, though mild irritability was noted in the early phase of treatment.

Dıscussion and Conclusion

The concurrent presentation of gender dysphoria in this twin pair is consistent with a genetic contribution, yet clinical differences between the cases reflect meaningful within-pair variation. The shared experience of a strict, single-parent upbringing may have influenced gender identity development, though the available data do not permit causal conclusions. A coordinated approach between psychiatry and endocrinology proved central to facilitating access to treatment and achieving symptom reduction in both cases. This report adds to a sparse literature and reinforces the importance of multidisciplinary care in this population.

References

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