Thursday 15 November 2007

Human chorionic gonadotropin

Human chorionic gonadotropin (hCG) is a peptide hormone produced in pregnancy, that is made by the embryo soon after conception and later by the syncytiotrophoblast (part of the placenta). Its role is to prevent the disintegration of the corpus luteum of the ovary and thereby maintain progesterone production that is critical for a pregnancy in humans. hCG may have additional functions, for instance it is thought that it affects the immune tolerance of the pregnancy. Early pregnancy testing generally is based on the detection or measurement of hCG. Because hCG is produced also by some kinds of tumor, hCG is an important tumor marker, but it is not known whether this production is a contributing cause or an effect of tumorigenesis.
Its primary role is to support the corpus luteum which secretes estrogen and progesterone. These hormones are necessary to support a pregnancy during the first trimester. hCG levels rise when pregnancy is established and it is the hormone measured by pregnancy urine test kits.

hCG products such as Profasi and Pregnyl are derived from human tissue. Ovidrel is a new pure product that is derived from mammalian cell DNA technology. It is injected subcutaneously facilitating patient administration. hCG is extensively used as a parenteral fertility medication in lieu of luteinizing hormone. In the presence of one or more mature ovarian follicles, ovulation can be triggered by the administration of hCG. As ovulation will happen about 40-45 hours after the injection of hCG, procedures can be scheduled to take advantage of this time sequence. Thus, patients who undergo IVF, typically receive hCG to trigger the ovulation process, but have their eggs retrieved at about 36 hours after injection, a few hours before the eggs actually would be released from the ovary. In a normal menstrual cycle, the release of LH is triggered when hormones (such as estrogen) reach the appropriate levels. This is governed by hormonal relationships mediated though the hypothalamic-adrenal-pituitary axis. As hCG supports the corpus luteum, administration of hCG is used in certain circumstances to enhance the production of progesterone.

In the male, hCG injections are used to stimulate the leydig cells to synthesize testosterone. The intratesticular testosterone is necessary for spermatogenesis from the sertoli cells. Typical uses for hCG in men include hypogonadism and fertility treatment. In the world of performance enhancing drugs, hCG is increasingly used in combination with various anabolic androgenic steroid (AAS) cycles. When AAS are put into a male body, the body's natural negative feedback loops cause the body to shut down its own production of testosterone via shutdown of the HPTA (hypothalamic-pituitary-testicular axis). High levels of AASs that mimic the body's natural testosterone trigger the hypothalamus to shut down its production of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Without GnRH the pituitary gland stops releasing luteinizing hormone (LH). LH normally travels from the pituitary via the blood stream to the testes where it triggers the production and release of testosterone. Without LH, the testes shut down their production of testosterone, causing testicular atrophy. In males, hCG mimics LH and helps restore / maintain testosterone production in the testes. As such, hCG is commonly used during and after steroid cycles to maintain and restore testicular size as well as endogenous testosterone production. However, if hCG is used for too long and in too high a dose, the resulting rise in natural testosterone will eventually inhibit its own production via negative feedback on the hypothalamus and pituitary.

During first few months of pregnancy, the transmission of HIV-1 from woman to fetus is extremely rare. It has been suggested this is due to the high concentration of hCG, and that the beta-subunit of this protein is active against HIV-1.

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