Background: Bone geometry is determined by sex steroids. While testosterone is thought to regulate periosteal apposition, estrogen regulates endocortical apposition. In adult transgender people, no change in bone geometry during gender-affirming hormone treatment (GAH) was observed (Wiepjes, Bone 2019). This might be because adults already achieved peak bone mass at start of GAH. The current study aims to investigate change in bone geometry in transgender adolescents who used puberty blockers (GnRHa) and subsequent GAH before peak bone mass was achieved. We hypothesized endocortical diameter to decrease in trans women and subperiosteal width to increase in trans men.

Methods: Transgender adolescents treated with GnRHa and GAH at our center were eligible for inclusion. DXA scans including Hip Strength Analysis were made at start of GnRHa, start of GAH and 2 years after start of GAH. Mixed model analyses were performed to study differences in subperiosteal width and endocortical diameter.

Results: In total 298 people with 353 DXA scans were included in these first analyses. In trans women, subperiosteal width at start of GnRHa was 3.36cm (95%CI 3.01;3.71). At start of GAH, subperiosteal width decreased (-0.60cm, 95%CI -1.05;-0.15) as it did after 2 years of GAH (-0.13cm, 95%CI -0.68;0.43). Endocortical diameter was 3.01cm (95%CI 2.69;3.32) at start of GnRHa, after which it decreased with -0.52cm (95%CI -0.93;-0.12) at start of GAH. After 2 years, a change of -0.16cm (95%CI -0.66;0.34) was seen. In trans men, subperiosteal width at start of GnRHa was 3.20cm (95%CI 3.03;3.37) with a change of -0.39cm (95%CI -0.63;-0.15cm) at start of GAH. After 2 years, the change was -0.24cm (95%CI -0.63;0.14). Endocortical diameter at start of GnRHa was 2.83cm (95%CI 2.68;2.99). At start of GAH this was -0.33cm (95%CI -0.55;-0.11) less. After 2 years the change was -0.22 (95%CI -0.57;0.13cm).

Conclusion: Our preliminary results showed a decrease in subperiosteal width and endocortical diameter in both trans women as trans men after start of puberty blockers and after 2 years of GAH.