Bone health

Gender-affirming hormone and surgical treatments have implications for bone health. Most research has focused on the bone health of transgender, Two-Spirit and non-binary (TTNB) people who have estradiol and testosterone levels that align with typical cisgender (cis) levels. In contrast, there is little research into how low-dose hormone regimens, such as those often used to achieve non-binary embodiment goals, affect bone health. For everyone, adequate calcium, vitamin D and sex hormone levels are crucial for maintaining bone health and particularly critical after a gonadectomy [1–3].

Note

In this document, we use the language of TTNB people assigned male or female at birth (AMAB/AFAB) to highlight that not everyone who takes gender-affirming hormone therapy (GAHT) identifies as a trans man or trans woman. While most cited resources use the terms trans women or trans men, it is likely that studies categorized participants according to hormone status rather than self-described gender. 

As noted above, there is little research into how low-dose hormones affect bone health. Unless otherwise noted, cited research has focused on hormone protocols that aim to mirror typical cis levels.

Bone mineral density classification

The T-score compares a person’s bone mineral density (BMD) to that of an average young adult. It is commonly used to categorize bone density measurements for people who are over 50 or have gone through menopause [4,5]. The Endocrine Society [6] recommends using a “uniform Caucasian (nonrace adjusted) female normative database for all transgender individuals of all ethnic groups” over the age of 50, regardless of hormone status (pp. 546-547). 

Standard T-score reference ranges

The Z-score is recommended for people who are under 50 or have not gone through menopause. It compares a person’s BMD to the average for a population matched according to sex, race, and age [4]. A Z-score ≤ -2.0 should be interpreted as low bone density and considered “below the expected range for age” rather than using the language of osteopenia [7].

Interpreting Z-score

Because a Z-score is population matched and reflects what is commonly seen within a client’s age group, the Z-score may decrease while the client’s BMD increases (or vice versa). For example, if a person’s BMD does not increase at the same rate as an age matched population, their Z-score may go from 1.0 to 0.5 even as their BMD increases. 

Generally speaking, after puberty, BMD is expected to gradually decrease with age [8]. Therefore, a person’s Z-score may increase over time if their BMD is maintained at the same level.

Pre-treatment

Research shows that, before starting estrogen-based GAHT, TTNB people AMAB tend to have lower bone mineral density (BMD) than cis men [1,3,9–12]. The baseline prevalence of osteoporosis in this demographic (T-score ≤-2.5) ranges from 14% to 18% while rates of low bone density (Z-score ≤ -2.0) are estimated at 22% [1,12]. 

Factors that contribute to baseline differences 

Effect of estrogen-based GAHT on BMD

Estrogen-based GAHT generally maintains and, in some cases, improves bone density. While research has shown mixed results, it is generally accepted that BMD is maintained after initiation of estrogen-based GAHT [3,11,12], with one study showing statistically significant increases in Z-scores of the lumbar spine in the first 10 years of treatment [8]. 

Additionally, studies of TTNB patients on estrogen-based GAHT show that the level of estradiol in the blood is correlated with BMD. Higher levels (≥ 238-443 pmol/L) are associated with increased BMD, whereas lower levels (≤ 118 pmol/L) are associated with decreased BMD [2,3,8,11]. In contrast, neither luteinizing hormone (LH) nor testosterone suppression correlated with changes to bone density [8,11]. One study found that taking vitamin D supplements was associated with a higher BMD in the lumbar spine and femoral neck after one year of estrogen-based GAHT [14]. 

In a separate study, authors found that 40% (n = 23) of TTNB people AMAB with a history of a gonadectomy had low BMD (Z-scores ≤ -2.0). Risk factors associated with low BMD were older age, low serum estradiol levels, and intermittent use of estrogen-based GAHT [13]. Therefore, adequate hormone replacement therapy is a key component of bone health after gonadectomy [1].

Fracture risk

Initial studies looking at fracture risk have suggested TTNB people AMAB over 50 have a higher risk of fractures compared to cis men (4.4% compared to 2.4%) with no statistically significant increased risk compared to cis women [15]. While one research group has used a modified fracture risk assessment (FRAX) tool to estimate a fracture risk for this demographic [13], there are currently no validated tools to estimate fracture risk for TTNB people [12,13]. 

While one study found that smoking appeared associated with a higher fracture risk in TTNB people AMAB, this finding was not statistically significant [15]. Additionally, smoking cessation was not associated with improvements in bone density after one year of GAHT [14].

Pre-treatment

TTNB people AFAB generally have similar bone mineral density to cis women prior to initiating GAHT [3,10,11]. This is likely due to similarities in baseline activity levels, vitamin D levels, and muscle mass [3].

Effect of testosterone on BMD

Testosterone GAHT maintains bone density through aromatization and direct mechanisms. Testosterone converts partially to estradiol via the aromatase enzyme. This conversion produces circulating estradiol levels that preserve bone density in most individuals [2,12]. Testosterone GAHT also increases lean muscle mass, resulting in higher mechanical load on the bones and increased bone growth [1,11]. 

After ten years of GAHT, BMD at the lumbar spine remains stable while Z-scores increase [8]. TTNB people AFAB who began testosterone GAHT when they were > 40 years old had an increase in their BMD as well as their Z-scores, potentially due to testosterone aromatization replenishing estrogen deficiency prior to beginning GAHT [8]. 

People who start on testosterone over the age of 40 or after menopause may have improved BMD or BMD scores (i.e., T-score or Z-score) that correlate with increased estradiol levels from testosterone aromatization [1,3,8,10,11]. Similarly, testosterone GAHT may be protective for TTNB people who have had an oophorectomy by mitigating bone loss [1].

Fracture risk

Due to lower rates of osteoporosis with long term testosterone treatment, current data on fracture risk is limited. Some evidence suggests that fracture risk may be similar to that of pre-menopausal cis women, and lower than that of cis men [1,12,15,16].  This has been attributed to a lower rate of participation in sports that are associated with traumatic fractures in comparison to cis men in the same age bracket rather than a difference in BMD [15].

Screening 

The International Society for Clinical Densitometry (2019) recommends the following approaches to for screening BMD in TTNB adults [6]:

Additionally, screening with bone densitometry is recommended if a client:

The 2023 Canadian Osteoporosis Guidelines recommend BMD screening for:

However, the authors caution that treatment recommendations for osteoporosis and the tools used for calculating a fracture risk (FRAX or the Canadian Association of Radiologists and Osteoporosis Canada (CAROC) system) have not been validated for use in TTNB people [17]. Further research is needed to guide screening and treatment for this demographic [17]. 

Prevention

In addition to considering BMD screening for TTNB clients who meet the criteria outlined above, additional dietary and lifestyle factors should be reviewed.

Calcium and vitamin D

Adequate daily intake of calcium (1000-1200 mg daily) and vitamin D (800-1000 IU daily) through diet or supplementation [18–20]. 

Weight-bearing exercise 

Regular exercise is associated with maintaining or increasing bone mass. Exercise can include weight-bearing aerobic exercises (e.g., stair climbing, jogging), strength and resistance exercises (e.g., swimming, weightlifting, cycling), and multicomponent exercises (exercises that combine different movements, such as dancing and aerobics) [18,21]. While walking has health benefits and appears to maintain BMD, it does not appear to improve BMD [21]. 

Gender-affirming hormone therapy 

Hormones play a key role in BMD and bone health. As detailed in previous sections, low serum estradiol levels are associated with lower BMD scores for TTNB people, regardless of assigned sex at birth [2,3,8,11]. Clients who take no hormones or low-dose GAHT after a gonadectomy may benefit from BMD screening. 

Alcohol and tobacco 

Alcohol and tobacco use may negatively impact BMD. One meta-analysis found a non-statistically significant link between increased alcohol intake and a rising rate of osteoporotic fractures [22]. The same analysis, however, found that light drinkers had higher BMD than those who abstain from alcohol [22]. Lower bone density rates found in those who consume large amounts of alcohol may be linked with malnutrition and other (unmeasured) comorbidities [23]. It is likely that alcohol, when consumed within recommended amounts, has a limited effect on BMD. 

In contrast, a systematic review found that tobacco use is consistently associated with decreased bone mineral density [24]. Where appropriate, discuss alcohol consumption and smoking cessation with clients as part of gender-affirming relational practice, integrating trauma- and violence-informed care principles.

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23. Peel A, Jesudason D, Martin S, Wittert G. Association of alcohol and bone mineral density dependent on type of alcohol consumed. J Bone Miner Metab. 2023;41: 702–713. doi:10.1007/s00774-023-01450-x

24. AL-Bashaireh AM, Haddad LG, Weaver M, Kelly DL, Chengguo X, Yoon S. The effect of tobacco smoking on musculoskeletal health: A systematic review. J Environ Public Health. 2018;2018: 1–106. doi:10.1155/2018/4184190