MON-186 - Body Mass Index as A Predictor of Testosterone Maintenance Dosing in Transgender Men Receiving Injectable Therapy

Background:
Testosterone therapy is a key component of gender-affirming care for transgender men. Although guideline-recommended dose ranges for testosterone therapy exist, there is limited evidence to guide prediction of individual maintenance dose requirements within these ranges. As a result, clinicians rely on clinical judgement to estimate dose needs over time. This study aimed to identify clinical predictors of testosterone dose requirements to inform anticipated maintenance dosing in transgender men.

Methods:
We conducted a retrospective chart review (n=242) at a single-center transgender health endocrinology clinic in Toronto. Adult transgender men (≥18 years) initiating injectable testosterone cypionate or enanthate were included. Linear mixed-effects models assessed associations between testosterone dose and body mass index (BMI), body surface area (BSA), and weight, adjusting for age, smoking status, and obstructive sleep apnea. Predicted doses were generated across the observed reference ranges of each metric and stratified by achieved serum testosterone levels.

Results:
BMI demonstrated the strongest predictor of testosterone dosing, revealing the best model fit compared to BSA and weight. Testosterone doses increased modestly between 6 and 12 months and showed minimal change from 12 to 24 months, indicating that most patients reached a stable testosterone dose by 12 months. BMI-based formulas were derived to estimate testosterone doses required to achieve low and high target serum testosterone ranges. For the low testosterone range (7.6–13.9 nmol/L), the predicted dose was 0.82 × BMI + 25.4 mg, and for the high range (13.9–31.4 nmol/L), the predicted dose was 0.63 × BMI + 41 mg. At stable doses, menstrual suppression was routinely achieved, occurring in 212 of 234 participants (90.6%) at 12 months of follow-up. Testosterone-associated secondary erythrocytosis, defined as a hematocrit >0.52 or hemoglobin >180 g/L, occurred in 4 of 233 participants (1.7%) at 12 months of follow-up. Predicted maintenance doses were lower than commonly used empiric doses, aligning with cis-male and trans-male clinical practice guidelines that prioritize embodiment goals and safety rather than strict attainment of cis-male testosterone ranges.

Conclusion:
BMI is the strongest clinical predictor of testosterone dose requirements in transgender men receiving injectable therapy. A 12-month time point represents a clinically meaningful target for estimating long-term maintenance dose needs. These BMI-based predicted testosterone doses should not serve as starting doses, but may help guide anticipated dose stabilization. Together, these findings support standardized testosterone titration and provide an evidence-based framework to inform maintenance dosing in gender-affirming hormone therapy.

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