Background
Previous research has shown that bone mineral density (BMD) Z-scores decrease during puberty suppression in transgender adolescents, with incomplete catch-up after initiation of gender-affirming hormone treatment in transgender girls. Because the natural course of BMD development in this population is unknown, it is unclear to what extent factors besides medical treatment contribute to the observed decrease. In this study we use cross-sectional data of both transgender girls and boys without medical treatment to assess the natural course of BMD.
Methods
All people under the age of 25 years visiting the gender identity clinic of Amsterdam UMC, were included in this retrospective cross-sectional cohort study if a DXA scan was available before start of medical treatment. BMD Z-scores of the sex assigned at birth were calculated. The relationship between age and BMD Z-score was analysed using a linear regression model with lumbar spine BMD Z-score as main outcome, and age at time of DXA as independent variable. Individuals assigned male at birth (AMAB), and individuals assigned female at birth (AFAB) were analysed separately.
Results
In total 1076 DXA scans were included, of which 401 were in AMAB, and 675 in AFAB. Age at DXA scan ranged from 12 to 25 years. In AMAB, the mean BMD Z-score at the lumbar spine at age 12 was -0.07 (±0.81) and was significantly lower at age 25, -1.13 (±1.14), (per one year older BMD Z-score -0.09 (95%CI -0.12;-0.07)). In AFAB, the mean BMD Z-score at age 12 was 0.09 (±0.97), which was similar to that at age 25, 0.27 (±1.33) (per one year older BMD Z-score -0.00 (95%CI -0.03;0.02).
Conclusion
In AMAB aged 12-25 years, who did not yet receive medical treatment, an inverse relationship was observed between lumbar spine Z-scores and age. In contrast, Z-scores were not related to age in AFAB between age 12 and age 25. This implies that in addition to puberty suppression and subsequent hormone supplementation, other factors, i.e. lifestyle factors, affect BMD development in AMAB during medical intervention.