Endocrine Abstracts

Objective: The aim of the current study is to examine the effects of gender affirming hormone therapy in transgender persons on calcium and phosphate concentrations and their regulatory hormones.

Design: A prospective cohort study

Methods: This study investigated changes in calcium and phosphate concentrations and their regulatory hormones in trans women and trans men during the first year of hormone therapy. Trans women received estradiol and anti-androgens, while trans men received testosterone. Calcium, phosphate, PTH, FGF23 and 1,25diOHD were measured at baseline and after 1 year of hormone therapy. Calcium concentrations were corrected for serum albumin. Correlations between change in gonadal hormones and change in calcium- and phosphate and related hormone concentrations were studied.

Results: In 40 trans women [median age 29.1 years, 95% CI 25.7 to 45.4] calcium decreased with 1.7% [95% CI -2.8 to -0.6] and phosphate increased with 6.1% [95% CI 0.9 to 11.3]. PTH decreased with 8.9% [95% CI -17.2 to -0.6], whereas FGF23 and 1,25diOHD did not change in trans woman. In 40 trans man [median age 25.5 years, 95% CI 24.1 to 29.1] calcium increased with 1.2% [95% CI 0.1 to 2.3] and phosphate decreased with 9.7% [95% CI -14.9 to -4.5]. PTH decreased with 10.5% [95% CI -19.5 to -1.5] and FGF23 increased with 31.8% [95% CI 18.2-45.3], while 1,25diOHD concentrations did not change in trans man. In a pooled analysis, 1 SD change in testosterone concentrations [32.7 nmol/l] was related to an increase in calcium of 0.02 mmol/l [95% CI 0.01 to 0.04], a decrease in phosphate concentration of 0.06 mmol/l [95% CI -0.09 to -0.03] an increase of FGF23 concentrations of 18 U/ml [95% CI 6 to 31]. Changes in estradiol or LH concentrations were not related to any of the outcomes.

Conclusions: 1 year of hormone therapy resulted in a decrease in phosphate concentrations in trans men and an increase in trans women. For calcium only a subtle change was observed in an opposite manner as phosphate changes. The increase in testosterone levels correlated with the decrease in phosphate concentrations. Increased phosphate excretion through FGF23 secretion or increased bone formation, may mediate the effects of testosterone.