Medical Transition: What Changes in the Body
Hormone replacement therapy (HRT) is one of the main tools of medical transition. Through the administration of estrogen or testosterone, the body undergoes profound changes affecting body composition, metabolism, hematological profile, and numerous other functions. These changes also have implications in the debate about sports participation. This article examines, based on peer-reviewed literature, what actually changes and what remains unchanged.
Important Premise
Medical transition is an individual journey: not all trans people choose hormone therapy, and not all combine it with surgical interventions. Every change described in this article refers to documented effects of long-term hormone therapy under medical supervision, as indicated in the Endocrine Society guidelines (2017) and the WPATH Standards of Care version 8 (2022) [1][2].
What Changes with Hormone Therapy
Hormonal and Metabolic Profile
The most immediate change concerns the hormonal profile: estrogen or testosterone levels are brought into the typical range of the affirmed gender [1]. This triggers a cascade of systemic effects that significantly modify how the body functions.
Body Composition
Hormone therapy alters fat distribution and muscle mass. Trans women on estrogen therapy develop fat redistribution toward hips and thighs, with a reduction in muscle mass. Trans men on testosterone therapy experience the opposite effect: increased lean mass and fat redistribution toward an android pattern [4]. These changes are well documented and begin in the first months of treatment, stabilizing over 2-5 years [3].
Hemoglobin and Blood
Testosterone stimulates red blood cell production and increases hemoglobin levels; estrogen has the opposite effect. After hormone therapy, the hematological values of trans people approach significantly those typical of the affirmed gender [3]. This is one of the most complete and measurable changes.
Secondary Sex Characteristics
Estrogen induces breast development (generally modest cup size, but variable) [6], skin softening, and reduced hair growth. Testosterone causes voice deepening (irreversible), beard growth, increased body hair, and redistribution of facial fat [1]. These changes are among the most visible and socially significant.
Cardiovascular Risk
Hormone therapy modifies the cardiovascular risk profile. Estrogen may slightly increase thromboembolic risk, particularly in the first years and with certain formulations [5]. Testosterone can affect the lipid profile [7]. Regular medical monitoring is essential to manage these risks, which remain overall low in the treated population [8].
Skin and Hair
Testosterone tends to increase sebum production and may accelerate androgenetic alopecia in predisposed individuals. Estrogen makes skin thinner and softer, and may slow or halt hair loss [1].
Psychological Well-Being
A 2020 systematic review confirmed that hormone therapy is associated with a significant reduction in depression and anxiety levels, and an improvement in quality of life [10]. This finding is consistent with decades of clinical practice and represents one of the strongest arguments in favor of access to hormone therapy for those who need it.
What Does Not Change
Chromosomes
The karyotype (XX, XY, or other variants) is not modified by hormone therapy. However, chromosomes determine only part of sexual development: many of their functions are “overwritten” by hormonal effects on the rest of the body.
Skeletal Structure
The adult skeleton, including height, shoulder and pelvic width, and limb proportions, does not change significantly with hormone therapy. Bone density, however, is affected: monitoring is important, especially for trans women, to prevent osteoporosis [9].
Endogenous Reproductive Capacity
Hormone therapy strongly reduces fertility, but does not always eliminate it definitively. Gamete production (sperm or oocytes) is compromised, often irreversibly after prolonged treatment [8]. For this reason, guidelines recommend discussing fertility preservation options before starting therapy [1][2].
Internal Organs
Without surgical interventions, internal reproductive organs remain unchanged. The prostate in trans women tends to shrink under estrogen effects but does not disappear [8]. Preventive screening remains important.
An Overall Perspective
Hormone therapy does not transform the body completely, but the changes are profound and systemic. Blood, metabolism, body composition, skin, and secondary sex characteristics are substantially modified [3][4]. Some structural aspects (bones, chromosomes) remain unchanged. This complexity reflects the fact that biological sex is not a single parameter but a set of characteristics that can be aligned in various ways.
Research continues to document the long-term effects of hormone therapy, confirming its safety profile when administered under medical supervision and following international guidelines [8].