Involves social/cultural roles, considering a person as male, female, both, or neither.
Involves physical attributes of gender.
A person whose gender identity differs from their assigned sex at birth (not necessarily going under physical transition).
Person (binary or non-binary) who is assigned as male at birth but has a predominantly feminine gender identity.
A person that desires a physical transition to the sex that corresponds with their gender identity.
During their lifespan, humans undergo different kinds of changes, some of which are related to internal senses and might be variable or permanent, while others are results as consequences of external factors. These variations include a broad spectrum of personal features such as tastes, dreams, fears, and of course self-awareness.
Feminizing Hormone Therapy (FHT) is one of the pathways by which transfeminine people go under physical transition to the sex that corresponds with their gender identity. The purpose of FHT is to induce female secondary sex characteristics while suppressing/minimizing male secondary sex characteristics in patients[1–3]. Additionally, some studies suggest improvement in gender dysphoria and the overall quality of life of transexual patients after receiving hormone replacement therapy. The following is a brief description of how FHT works and some of the most relevant changes in the integumentary system.
How does FHT work?
The general approach of FHT involves the administration of anti-androgens combined with female hormones, such as estrogen, and in some cases progestogen1. Estrogen administration reduces androgen production by inhibiting gonadotropin secretion from the pituitary gland . In parallel, anti-androgens present different mechanisms of action, among which the antagonistic effect on androgen receptors predominates, leading to a decrease in androgens activity [6,7]. These alterations in hormone levels induce feminizing physical changes in the human body , i.e. redistribution of subcutaneous fat, reduction of body hair, change in sweat/odor patterns, and others. Nevertheless, undesirable effects may occur and must be considered.
FHT and hair follicle
Decreased facial and body hair growth
Hormones, especially androgens, such as testosterone and dihydrotestosterone (DHT), play a key role in the hair follicle structure and its growth cycle. They bind to intracellular receptors located in the dermal papilla cells of the hair follicle and trigger biological reactions, leading to the conversion of small, straight, hair into larger darker terminal hair . FHT leads to facial and body hair diameter reduction appearing from the 4th month of treatment , followed by thinner follicle structure and slow growth over the years.
However, it is important to highlight that FHT generally does not cause cessation of facial hair growth. Furthermore, facial hair and physiological changes caused by FHT can lead to undesirable skin conditions. Estrogen and anti-androgen administration is related to decreased sebum production, which may lead to pruritus and folliculitis and subsequent complications such as scarring, dark spots, and skin damage.
Thus to prevent unwanted outcomes, additional physical alternatives should be considered. Electrolysis and laser are currently used treatments to reduce the density of facial hair shafts [12,13].
The other side of the coin: what about Androgenetic Alopecia?
Androgenetic alopecia (AA) is characterized as a form of hair loss from the scalp, that occurs in a progressive and predictable pattern. Although the underlying mechanism is not completely elucidated yet, it is generally accepted that AA is associated with high levels of DHT, in a person with a genetic predisposition to it.
In AA, testosterone, and high levels of DHT are responsible for alteration in the hair follicle cycle, leading to progressive miniaturization of the follicle structure[15,16]. A clinical case  exhibited scalp hair regrowth in a transfeminine patient undergoing FHT after six months of oral estradiol and spironolactone therapy, suggesting that this outcome is correlated with the diminishing of total testosterone levels due to FHT. Certainly, it is worth considering and assessing the reproducibility of this result, attempting to establish effects linked to existing therapies.
FHT and skin
Melasma is a skin pigmentary condition, distinguished by symmetrical hyperpigmented patterns. This disorder is associated with a genetic predisposition, sunlight exposure, and triggering factors required for its development, such as hormonal alteration.
Female hormones stimulate melanin production -the pigment responsible for skin color and UV protection-, which along with crosstalk between different skin cells -keratinocytes, fibroblasts, and mast cells- leads to a hyperpigmentation progression pattern. Due to hormone administration, transfeminine patients undergoing FHT have a higher risk of developing melasma. To prevent hyperpigmentation of affected skin, rigorous use of sunscreen and photoprotection is required.
Overall, hormone administration leads to physical changes in the human body, including desirable and undesirable effects that should be taken into consideration. In the same manner, understanding dermatological alterations as a consequence of FHT -higher risk of skin pigmentation, or skin more prone to itching and folliculitis-, elucidates the landscape. In this way, both clinicians and patients are aware of possible difficulties and can approach diverse alternatives to avoid skin damage. For instance, mandatory and regular use of sunscreen, and developing a skincare routine based on specific needs.
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