Sex change is gender reassignment therapy, that is, all medical procedures transgender people can have, or specifically to sexual reassignment surgery, which usually refers to genitalia surgery only. It is also sometimes used for the medical procedures intersexual people undergo, or, more often, are subjected to as children.
"Sex change" is sometimes also used for the whole process of changing gender role and the medical procedures associated with it. Since changing of gender role, i.e. living as a woman instead of living as a man, or living as a man instead of living as a woman, is much more important to almost all transgender people than any medical procedures, this use is even more inaccurate. (Of course, medically induced changes and surgeries are often needed to make a change of gender role at all possible, both socially and legally. Also, they can have a very significant impact on the well-being of people having them.)
Sex change in animals 
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Some species exhibit sequential hermaphroditism. In these species, such as many species of coral reef fishes, sex change is a normal anatomical process. Clownfish, wrasses, moray eels and other fish species are known to change sex including reproductive functions. A school of clownfish is always built into a hierarchy with a female fish at the top. When she dies, the most dominant male changes sex and takes her place. In the wrasses (Family Labridae), sex change is from female to male, with the largest female of the harem changing into a male and taking over the harem upon his disappearance.
Sex change in humans 
Sex change is a term often used for sex change in humans, that is all medical procedures transgender people may pursue, or specifically to sex reassignment surgery, which usually refers to genital surgery only. It is also sometimes used for the medical procedures intersex people undergo, or, more often, are subjected to as children. "Sex change" is sometimes also used for the whole process of changing gender role and the medical procedures associated with it. Since changing of gender role, i.e. living as a woman instead of living as a man, or living as a man instead of living as a woman, is much more important to almost all transgender people than any medical procedures, this use is even more inaccurate. (Of course, medically induced changes and surgeries are often needed to make a change of gender role at all possible, both socially and legally. Also, they can have a very significant impact on the well-being of people having them.)
- Gonads (Ovaries and/or testicles)
- Hormone status
- Primary sex characteristics, sometimes also secondary sex characteristics
Not all of these factors can be changed, however:
- Chromosomes cannot be changed.
- Gonads can be removed, but not replaced
- Hormone status is easily changed
- Existing secondary sex characteristics can to some extent be changed; existing ones mostly through surgery, non-existing ones can be induced to grow through hormones.
For example: Changing a male genital anatomy into a good or even excellent female appearance with functioning is complicated, but entirely possible; changing a female genital anatomy into an even reasonably male appearing one however is extremely complicated and not successful very often; function is always limited.
Gender reassignment is usually preceded by a period of feminization or masculinization. This is accomplished by the use of either synthetic/natural hormones (such as estrogens, androgens, progestogens and antiandrogens). The most common period before gender reassignment surgery is two years, as specified by the Standards of Care for the Health of Transsexual, Transgender, and Gender Nonconforming People. Body changing hormones are normally started after sufficient counselling, or a period of living 'full-time' (in the target gender) for a minimum of six months. Transgenders from all over the world become medical tourist and fly to other countries to have their surgeries because they simply can't afford it back home. They save over 70% by travelling to Mexico, Thailand, India and Serbia.
Natural sex change in humans 
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Several medical conditions can result in a natural sex change in humans, where the appearance at birth is somewhat, mostly, or completely of one sex, but changes over the course of a lifetime to being somewhat, mostly or completely of the other sex. The overwhelming majority of natural sex changes are from a female appearance at birth to a male appearance after puberty, due to either 5-alpha-reductase deficiency (5alpha-RD-2) or 17-beta-hydroxysteroid dehydrogenase deficiency (17beta-HSD-3). A relative handful of male to female changes have been reported, and the etiologies of these are not well understood.
Genetic females (with two X chromosomes) with congenital adrenal hyperplasia lack an enzyme needed by the adrenal gland to make the hormones cortisol and aldosterone. Without these hormones, the body produces more androgen, a type of male sex hormone. This causes male characteristics to appear early (or inappropriately).
Genetic males (with one X and one Y chromosome) with androgen insensitivity syndrome is resistant to male hormone androgen. As a result, the person has some or all of the physical characteristics of a woman, despite having the genetic makeup of a man. The degree of sexual ambiguity varies widely in persons with incomplete AIS. Incomplete AIS can include other disorders such as Reifenstein syndrome which is associated with breast development in men.
See also 
- Black, MP; Grober, MS (2003). "Group sex, sex change, and parasitic males: Sexual strategies among the fishes and their neurobiological correlates". Annual review of sex research 14: 160–84. PMID 15287162.
- "Nemo Meets Neuroscience" (Article). Beckman Institute. Retrieved 2011-03-21.
- Pfäfflin F, Junge A (2003). Sex reassignment thirty years of international follow-up studies after sex reassignment surgery: a comprehensive review, 1961–1991. International journal of transgenderism. Symposion: Düsseldorf. OCLC 244295488.[page needed]
- Cohen-Kettenis, Peggy T. (2005). "Gender Change in 46,XY Persons with 5α-Reductase-2 Deficiency and 17β-Hydroxysteroid Dehydrogenase-3 Deficiency". Archives of Sexual Behavior 34 (4): 399–410. doi:10.1007/s10508-005-4339-4. PMID 16010463.
- Bertelloni, Silvano; Maggio, M. Cristina; Federico, Giovanni; Baroncelli, Giampiero; Hiort, Olaf (2006). "17β-Hydroxysteroid dehydrogenase-3 deficiency: A rare endocrine cause of male-to-female sex reversal". Gynecological Endocrinology 22 (9): 488–94. doi:10.1080/09513590600921358. PMID 17071532.
- Salt D, Brain Z (June 2007). "Intersex: Case studies". Cosmos (15).
- Khan U (2008-07-02). "Father of five naturally turning into a woman". Daily Telegraph UK.
- U.S. National Library of Medicine, Congenital adrenal hyperplasia
- Bostwick, J. M.; Martin, K. A. (2007). "A Man's Brain in an Ambiguous Body: A Case of Mistaken Gender Identity". American Journal of Psychiatry 164 (10): 1499–505. doi:10.1176/appi.ajp.2007.07040587. PMID 17898338.
- U.S. National Library of Medicine, Androgen insensitivity syndrome