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The female sex hormones are produced mainly in the ovaries, in pregnant women also in the placenta. They are carried to their target cells by the bloodstream.
To ensure that the estrogens and progesterone are available at exactly the right time and in the required amounts production of the sex hormones is controlled by the hypothalamus (a part of the forebrain) and the pituitary gland.
The highest level of control is the hypothalamus, which sends a releasing hormone to the pituitary gland. In response to the releasing hormone the pituitary sends gonadotropins (hormones which control the function of the gonads or sex glands) to the ovaries. This leads to production of sex hormones by the ovaries. The sex hormones are released into the bloodstream and carried to their target organs as well as to the hypothalamic-pituitary system.
Depending on the concentrations of the sex hormones in the blood the activity of the hypothalamic–pituitary system is increased or reduced. High blood levels of estrogens and progesterone suppress the release of gonadotropins, while low levels stimulate the pituitary to step up production. Understanding this so-called 'feedback principle' is also the basis for understanding how the pill and other hormonal methods of contraception work.
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