Hormone replacement therapy (HRT) for transgender and transsexual people replaces the hormones naturally occurring in their bodies with those of the other sex. However, not all cases of hormone replacement therapy are used by transgendered people. Some reasons for this include men who wish to have a hair-free body, as a result of less of the testosterone androgens in their body. Its purpose is to cause the development of the secondary sex characteristics of the desired gender. It cannot undo the changes produced by the first natural occurring puberty of transgender people, this is done by sexual reassignment surgery and for transwomen by epilation. Some intersex people also receive HRT, either starting in childhood to confirm the gender they were assigned, or later, if this assignment has proven to be incorrect.
While some people argue that hormonal therapy does not truly masculinize or feminize, the question is one of definitions. If by masculinize and feminize one means to completely reproduce the male or female biological state, that cannot be done with current medical or surgical therapy. However, the goal of HRT, and indeed all somatic treatments, is to provide patients with a more satisfying body that is more congruent with their true psychological gender identity. It should be noted that the effects of hormonal therapy are often much more satisfying to transgender men than transgender women. It is easier to produce secondary male sexual characteristics with androgens than it is to rid transgender women of those established characteristics.

Formal requirements for HRT
The requirements for hormone replacement therapy vary immensely, often at least a certain time of psychological counselling is required, and so is a time of living in the desired gender role, if that is at all possible, in order to assure that they can psychologically function in that gender role. This period is sometimes called the Real Life Experience (RLE). See also Standards of care for gender identity disorders.
Some individuals choose to self-administer their medication ("do-it-yourself"), often because available doctors have too little experience in this matter, or no doctor is available in the first place. Sometimes, trans persons choose to self-administer because their doctor will not prescribe hormones without a letter from the patient's therapist stating that the patient meets the diagnostic criteria for GID and is making an informed decision to transition. Many therapists require at least 3 months of continuous psychotherapy and/or a real life test in order to write such a letter as is suggested in the HBIGDA Standards of Care. In these circumstances, the individual may self-administer until they can get these authorizations, feeling that they shouldn't have to wait for a medical professional to be convinced of their situation. In addition, as many individuals must pay for evaluation and care out-of-pocket, expense can also be prohibitive to pursuing such therapy.
However, self-administration of hormones is potentially dangerous. Individuals seeking physicians who are knowledgeable and willing to treat transgender patients may wish to consult transgender support groups or a directory of LGBT-friendly doctors, in the USA for example the Gay and Lesbian Medical Association's referral service at GLMA.org.

Changes established at puberty
A number of skeletal and cartilaginous changes take place after the onset of puberty at various rates and times. Sometime in the late teen years epiphyseal clusure (in other words, the ends of bones are fused closed) takes place and the length of bones is fixed for life. Consequently total height and the length of arms, legs, hands, and feet are not affected by HRT. However, details of bone shape change throughout life, bones becoming heavier and more deeply sculptured under the influence of testosterone. Many of these differences are described in the Desmond Morris book Manwatching.
Pelvis: The pelvis in females tends to be wider than in males and tilted forward; the pelvis in males tends to be more circular and tilted upwards.

Hands: Male hands and feet tend to be larger than female hands and feet in persons of equal height.

Upper Arm: The upper arm in females tends to be significantly longer (about 1") than in males of the same height.

Head: Females tend to have smaller heads than males of the same height.

Chest: Female ribcages tend to be narrower than those of males in the same height.

Facial changes develop gradually over time, and sexual dimorphism (physical difference between the sexes) tends to increase with age. Within a population of similar body size and ethnicity:

• Brow: Males tend to develop heavier bony brows than females.

• Cheeks: Female cheeks tend to be fuller and more rounded. Under the influence of estrogen, fat is deposited beneath the skin and overall facial and body contours become softer.

• Nose: The tips of the nasal bones tend to grow more in males than females, creating a larger (longer or wider) nose.

• Jaw: The jaw in males tends to grow wider and more deeply sculptured than in females.

• Larynx: At puberty, the bones and cartilage of the voicebox tend to enlarge less in females than males. In some males, the larynx becomes visible as a bony "adam's apple."

• Lips: Females tend to have thicker, fleshier lips than males of the same size.