The Role of Hormones in Growth and Development
Height development in adolescents is intricately linked to hormonal changes during puberty. Sex hormones influence growth spurts and the timing of growth plate fusion in long bones. The effectiveness and impact of oestrogen and testosterone on bone development are influenced by the timing, dosage, and individual response to the hormone therapy.
Testosterone stimulates the growth and strengthening of bones and, through its conversion to oestrogen, plays a role in the closure of growth plates. Oestrogen contributes to both the growth in height and the attainment of peak bone mass.
Long term use of puberty blockers without the introduction of sex hormones requires balancing the mental and emotional benefits of gender-affirming treatment with the need for normal physical, sexual, and psychological development. This is done with regular monitoring and personalised care.
Delaying gender-affirming hormone therapy can affect the final height of transgender individuals, potentially leading to taller stature in transfeminine individuals and shorter stature in transmasculine individuals than might be desired.
The Role of Testosterone
- Stimulation of Bone Growth: Testosterone directly stimulates the growth of bone tissue. It contributes to the increase in bone size and density, which is a hallmark of puberty. This is particularly evident in the rapid growth of long bones, contributing to the height increase seen during puberty.
- Conversion to Oestrogen: Testosterone converts to oestrogen through a process called aromatisation. Oestrogen closes the growth plates (epiphyseal plates) in bones. The closure of these plates ends height growth.
- Muscle Growth and Bone Health: Testosterone also promotes muscle growth and strength. Increased muscle mass places greater mechanical load on bones, which in turn stimulates bone formation and strengthening. This interaction between muscles and bones is important for overall skeletal health.
- Bone Density: Testosterone contributes to peak bone mass attainment. Higher peak bone mass during adolescence is associated with a reduced risk of osteoporosis and bone fractures later in life.
- Regulation of Bone Resorption: Testosterone helps regulate bone remodelling by balancing bone formation and resorption (the process by which bone is broken down). This maintains healthy bone structure and density.
- Differential Effects Based on Gender: In transgender adolescents undergoing masculinising hormone therapy, the introduction of testosterone contributes to increased bone density and changes in bone structure that align with male puberty patterns. The timing and dosage of testosterone affect the outcomes in bone development.
Testosterone and Height in Transmasculine Youth
- Importance of Timing: For transgender boys and transmasculine individuals, starting testosterone therapy at the appropriate time can maximise height potential.
- Avoiding Premature Closure: If testosterone therapy starts off too high, it might accelerate growth plate fusion, potentially limiting height.
- Post-Puberty Considerations: Once natural puberty has led to the fusion of growth plates, additional height gain becomes unlikely.
The Role of Oestrogen
- Growth Spurt Initiation: Oestrogen starts the growth spurt associated with puberty. It stimulates the lengthening of long bones, contributing to the increase in height.
- Closure of Growth Plates: Oestrogen eventually closes growth plates (epiphyseal plates) in long bones. This marks the end of the bone growth in length and, the end of height increase.
- Bone Density: Oestrogen enhances the activity of osteoblasts (cells that form new bone) and decreases the activity of osteoclasts (cells that break down bone).
- Regulation of Bone Remodelling: Oestrogen balances bone resorption and formation. This maintains bone strength and integrity.
- Impact on Skeletal Structure: Oestrogen influences the shape and structure of the skeleton. In individuals undergoing feminising hormone therapy, oestrogen leads to changes that align more closely with typical female skeletal development, such as a broader pelvis.
- Prevention of Bone Loss: Oestrogen has a protective effect against bone loss. Low oestrogen levels, as seen in people on puberty blockers for too long without oestrogen, postmenopausal women or in conditions like amenorrhea, are associated with decreased bone density and increased risk of fractures.
Oestrogen and Height in Transfeminine Youth
- Managing Oestrogen Therapy: It not only induces feminising physical changes but also regulates height.
- Counteracting Testosterone-Induced Growth: Natural testosterone production during puberty can lead to increased height. Maximising oestrogen therapy at the right time can help limit this height gain, aligning development more closely with cisgender female patterns.
- Balanced Approach: The goal is to administer oestrogen in a way that supports both feminisation and desired height outcomes, without introducing it so early that it compromises either goal.
The Impact of Puberty Blockers
The use of puberty blockers in adolescents halts the development of undesired secondary sexual characteristics. Puberty blockers should not be used for an extended period without the subsequent introduction of sex hormones.
- Bone Health: Puberty is a key period for bone development and mineralisation. During this time, people usually gain a significant portion of their bone mass, setting the foundation for bone health in adulthood. Puberty blockers delay this process, potentially leading to lower bone density and an increased risk of osteoporosis later in life. Introducing sex hormones resumes normal bone development.
- Psychological Impact: Extended use of puberty blockers without introducing appropriate sex hormones can also have psychological and emotional impacts. Puberty is not only a time of physical development but also of emotional and social maturation. Delaying this process for too long without a transition plan can lead to feelings of stagnation and social isolation from peers who are progressing through their developmental stages.
- Sexual and Reproductive Health: Puberty is a time when sexual and reproductive organs mature. Long-term use of puberty blockers can impact the development of these organs, which may have implications for sexual health and, potentially, fertility in the future.
- Growth and Development: Puberty blockers can be beneficial in aligning physical appearance with gender identity, but prolonged use without the introduction of sex hormones can impact overall growth and development. This includes aspects like height, muscle mass, and fat distribution, which are influenced by sex hormones.
- Preparation for Adult Health: Delaying puberty indefinitely can impact a young person's readiness for adult health responsibilities.
Delaying Gender-Affirming Care
Delaying gender-affirming care can impact the final height of transgender girls (transfeminine) and transgender boys (transmasculine).
Impact on Transgender Girls
- Natural Testosterone Effects: If gender-affirming care is delayed for transgender girls, the testosterone present in their body during natural puberty will continue to exert its effects. This includes the typical male growth spurt, which can result in a final height that is taller than the average for cisgender females.
- Bone Growth and Structure: Testosterone during natural male puberty also influences bone structure, such as broader shoulders, longer limbs and larger hands and feet, and a taller stature.
- Puberty Blockers and Oestrogen Therapy Timing: The timely use of puberty blockers and subsequent oestrogen therapy can help prevent these changes by limiting the influence of testosterone on growth. Delaying treatment reduces the opportunity to influence these developmental processes.
Impact on Transgender Boys
- Natural Oestrogen Effects: In transgender boys, the presence of oestrogen during natural female puberty promotes a growth pattern typical of cisgender females. This usually results in a shorter final adult height compared to cisgender males.
- Closure of Growth Plates: Oestrogen leads to the earlier closure of growth plates in bones. Delaying testosterone therapy means that the growth plates may close earlier, limiting height potential.
- Testosterone Therapy and Growth Potential: Starting testosterone therapy before the closure of growth plates can lead to a growth pattern more aligned with male puberty, which might include an increase in height. If gender-affirming care is delayed until after these plates have closed, this opportunity is reduced.
Individual Variation
There is significant individual variation in response to hormone therapy. Genetic factors, the timing of puberty onset, and the stage of bone maturation at the start of hormone therapy all play a role in height and bone growth.
Puberty Blockers: A Strategic Tool
Puberty blockers are often used as an initial step in gender-affirming treatment. They delay the onset of puberty, extending the period during which growth plates remain open.
- Flexibility in Growth: This delay can provide additional time for height growth, especially beneficial for transgender youth who start their transition before or early in puberty.
- Tailored Hormone Introduction: After an appropriate period on puberty blockers, the introduction of gender-affirming hormones (testosterone or oestrogen) can be timed to optimise both physical development and height outcomes.