OR27-04 Risk Factors For Low Baseline Bone Mineral Density In Gender Diverse Youth

Background Sex steroids such as testosterone and estrogen are necessary for accumulation of bone mass. Transgender youth treated with gonadotropin releasing hormone analogues (GnRHa) to block natal puberty for gender-affirming care are at risk of low bone mineral density (BMD). Previous studies indicate that transfemale patients assigned male at birth (AMAB) have low BMD at baseline, during and after GnRHa treatment despite cross hormone treatment. Transmales assigned female at birth (AFAB), however, have normal BMD at baseline that decreases upon GnRHa treatment, with normalization upon cross hormone therapy. The reason(s) for the low baseline BMD in transfemales is unclear. We aimed to assess the baseline characteristics of transgender youth at a single multidisciplinary gender clinic prior to medical intervention and determine factors associated with BMD. Methods This is a retrospective chart review of patients <19 years old evaluated in the gender clinic. Dual-energy x-ray absorptiometry (DXA) scans were obtained prior to initiation of GnRHa or cross-hormone therapy per Endocrine Society guidelines for the treatment of gender dysphoria. We included patients with DXA scans completed prior to initiation of treatment with GnRHa or cross gender hormones and excluded those with concurrent medical diagnoses that may affect bone density. Data collected were bone mineral density (BMD) Z-scores, anthropometric data, vitamin D and calcium levels, and calcium intake. Multivariable linear regression models were used to assess the impact of vitamin D levels, height Z-score, weight Z-score, and BMI Z-score on subtotal body BMD Z-score, adjusted for sex assigned at birth and age. Results Sixty-four patients were included in our analysis. Of these, 73% were AMAB and 27% AFAB. Gender identity was male in 14%, female in 44%, and non-binary in 42%. Average height Z-score was 0.12, weight Z-score 0.27, and BMI Z-score 0.22 (using sex assigned at birth). Subtotal body BMD Z-scores were greater than zero in 11%, between zero and greater than -2 in 59%, and less than or equal to -2 in 30% of tested patients. AMAB patients had lower BMD Z-scores compared to those AFAB (p<0.05 for all Z-scores). There was a positive association with BMI, height, and weight Z-scores and increasing BMD Z-scores after adjusting for sex assigned at birth and age (p<0.05 for all Z-scores). Patients who consumed <2 servings of calcium per day had lower BMD Z-scores (p<0.05 for all Z-scores). Average vitamin D level was 24 ng/ml (+/- 9.5 SD) with no significant association with BMD Z-scores (adjusted for sex assigned at birth). Conclusions Patients AMAB and patients with calcium intake of < 2 servings/day are associated with lower baseline BMD in a cohort of adolescents seen in a multidisciplinary gender clinic. Height, weight, and BMI are associated linearly with BMD Z-score, following patterns previously described in other populations.