Background: Pubertal growth and adult height are important to many transgender adolescents undergoing medical transition. However, few studies are available on the impact of puberty suppression (PS) with GnRH analogues and hormonal therapy (HT) with testosterone on growth in transgender youth.

Objectives: To evaluate the effect of PS and HT on growth and adult height (AH) in trans boys (female sex at birth, male gender identity).

Methods: A total of 147 trans boys seen at the gender identity clinic of the VU University Medical Center between 1998 and December 2018 who reached AH were included. Data on height, bone age (BA) and parental height were collected. SDS was calculated based on female references. Midparental height was calculated and predicted adult height (PAH) was determined according to Bailey & Pinneau. Outcomes were compared between those with growth potential (BA <14 years at start PS or, if BA was not available, menarche <1 year before start of PS, n=62) and those with little/no growth potential (n=85).

Results: In those with growth potential, PS resulted in a decrease of height SDS from 0.1 ± 1.5 at baseline to -0.1 ± 1.0 at start HT, and a significant decrease of bone maturation. During HT, height SDS significantly increased by 0.3 SDS (95% CI 0.2 to 0.5). The difference between height SDS at start PS and AH was 0.1 (95% CI -0.2 to 0.5). Mean AH was 172.0 ± 6.8 cm (0.2 ± 1.1 SDS), which was 4.0 ± 6.0 cm above midparental height and 3.0 ± 3.6 cm above PAH at start of PS. A younger BA at start PS resulted in an AH significantly further above PAH (1.2 cm/year; 95% CI 0.4 to 2.1). Similarly, a younger BA at start HT was associated with an AH further above PAH (1.3 cm/year; 95% CI 0.6 to 2.1). Subjects with little/no growth potential reached a mean AH of 169.0 ± 6.8 cm (female height SDS -0.3 ± 1.1) which was 3.0 ± 4.7 cm above midparental height.

Conclusion: Height SDS at AH was similar to that at baseline. However, adolescents reached a larger AH than PAH at start PS. This effect was greater in those who started treatment at a younger BA.