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.
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