Reversible Effects of Puberty Suppression on Bone Strength, Mass, and Body Composition in Adolescent Mice After Testosterone Therapy

To the Editors:

Concern regarding the deleterious bone effects of puberty suppression for transgender and gender diverse (trans) youth is an issue for patients, their families, and treating clinicians. Although there is considerable variability in access across jurisdictions, there has been an increase in demand for gender-affirming care for trans youth. Pubertal suppression with gonadotropin releasing hormone antagonist or agonist (GnRHa) therapy is typically commenced in early puberty (ie, Tanner stage 2) and aims to delay pubertal progression to allow maturation of the individual until an appropriate time for possible masculinizing or feminizing gender-affirming hormone therapy. Masculinizing hormone therapy typically involves testosterone therapy in standard doses used for hypogonadal men to achieve testosterone concentrations in the typical male reference range.

There are few studies in humans on the impact of GnRHa on bone. Retrospective and small uncontrolled cohort studies in trans youth have shown that even prior to GnRHa, bone mineral density (BMD) measured by dual-energy X-ray absorptiometry (DXA) may be lower than in age-matched youth.^(1-3) This is largely determined by factors such as lower body mass index, low vitamin D, suboptimal calcium intake, and lower physical activity.^(1-3) After GnRHa treatment, BMD declines, but there are also associated increases in body fat including in bone marrow adipose tissue.⁽⁴⁾ After gender-affirming hormone therapy, BMD Z-scores increase but may well remain below age-matched peers.^{(5, 6)} However, all existing studies have used DXA, which has poor precision and low correlation with fracture.⁽⁷⁾ Additionally, body fat increase with GnRHa influence photon attenuation which may artifactually underestimate BMD.⁽⁸⁾ Findings also vary depending on which reference range (male or female) is used and it is unknown whether timing of gender-affirming hormone therapy commencement matters.⁽⁹⁾ Additionally, it is difficult to extrapolate whether such changes impact peak bone mass accrual or whether observed changes are associated with a long-term risk of bone fragility or fracture in trans youth.

Although methodology to precisely measure the microarchitecture or breaking strength of bones in humans is challenging, this month's issue of the Journal of Bone and Mineral Research (JBMR) publishes a clinically relevant translational study which overcomes many of the limitations in existing literature.

Mimicking the clinical treatment regimens typically used for trans boys (assigned female at birth), Dubois and colleagues⁽¹⁰⁾ developed a mouse model to understand the impact of early puberty suppression on body composition, bone mass, and bone strength.

Prepubertal 4-week-old female mice were treated with degarelix as the GnRH antagonist which was followed by either placebo silastic implants, testosterone therapy from 6 weeks (early puberty), or testosterone therapy from 8 weeks (late puberty) onward. At the 16-week mark, considered early adulthood, outcomes were compared to untreated male and female mice.

Degarelix treatment significantly increased fat mass accumulation (restricted to white adipose tissue) and despite reduced food intake, fat mass was almost twofold higher than control animals of either sex.⁽¹⁰⁾ Degarelix also reduced lean mass and grip strength. Within 2 weeks of testosterone replacement (regardless of whether it was initiated early or late) body composition and bone marrow adiposity reversed and were similar to male controls. Grip strength was restored to female levels.

As expected, bone microarchitecture was sexually dimorphic with lower trabecular bone volume fraction (BV/TV) and cortical bone mass assessed by micro-computed tomography (μCT) at the femoral metaphysis in female compared with male mice. Degarelix reduced trabecular bone volume, cortical bone mass, and femoral bone breaking strength to levels lower than both male and female controls.⁽¹⁰⁾

Reassuringly, the negative effects of degarelix were counteracted by testosterone therapy. BV/TV increased, mostly due to an increase in trabecular number and decrease in trabecular separation, because effects on trabecular thickness were marginal.⁽¹⁰⁾ The duration of GnRHa therapy and the timing of testosterone administration was significant on the magnitude of increase in trabecular BV/TV. Testosterone replacement commenced in early puberty, increased BV/TV to the level of male controls by adulthood; however, if commenced in late puberty, BV/TV only increased to the level of female controls. Given that peak bone mass in mice is achieved at 4–6 months of age,⁽¹¹⁾ it remains unclear if a longer duration of testosterone therapy may further increase volumetric bone density. Cortical bone mass and femoral bone breaking strength was restored with testosterone up to female levels regardless of timing of testosterone replacement. Bone length was unaffected by GnRHa or testosterone therapy.

Although there are physiological differences between mice and humans, it is reasonable to hypothesize that for trans boys, fracture risk will not be increased after testosterone therapy provided that lifestyle factors are optimized for bone health (adequate vitamin D, physical activity, etc). Although the duration of GnRHa therapy is often difficult to control clinically (dependent upon when a patient presents for care, ease of access to GnRHa and readiness for hormonal therapy), earlier commencement of testosterone is more beneficial from a bone and body composition perspective. Certainly in adult trans men who have been on established testosterone therapy, it appears as though bone microarchitecture is preserved⁽⁸⁾ and no increased risk of fracture is seen when compared to cisgender individuals.⁽¹²⁾

This clinically relevant translational study is reassuring for trans boys who are treated with GnRHa followed by testosterone therapy. Further studies are needed in humans, particularly in trans girls (assigned male at birth) and trans women who may have compromised bone structure⁽⁸⁾ and further mouse models in trans girls are awaited.

Ada Cheung: Conceptualization; funding acquisition; writing – original draft; writing – review and editing.

The peer review history for this article is available at https://publons.com/publon/10.1002/jbmr.4906.

ASC has received product (estradiol and progesterone) for investigator-initiated trials from Besins Healthcare. Besins Healthcare have not provided any monetary support nor had any input into the design and analysis of research studies or the writing of any manuscripts.