|March 26, 2016|
Human chorionic gonadotropin (hCG) is a peptide hormone produced in pregnancy, that is made by the embryo soon after conception and later by the trophoblast (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.
The drugs Pregnyl®, Follutein®, and Ovidrel® use chorionic gonadoptropin as the active ingredient in their product. These preparations are used in assisted conception in lieu of luteinizing hormone to trigger ovulation.
hCG is a glycoprotein composed of 237 amino acids with a molecular mass of 36.7 kDa. It is heterodimeric, with an α (alpha) protein subunit|subunit identical to that of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). Its β (beta) subunit is unique to hCG.
hCG promotes the maintenance of the corpus luteum and causes it to secrete the hormone progesterone. Progesterone enriches the uterus with a thick uterine lining|lining of blood vessels and capillary|capillaries so that it can sustain the growing fetus.
Because of its similarity to Luteinizing hormone|LH and Follicle stimulating hormone|FSH, hCG can also be used clinically to induce ovulation in the ovary|ovaries as well as testosterone production in the testes. As the most abundant biological source is women who are presently pregnant, some organizations collect urine from gravidae to extract hCG for use in Assisted reproductive technology|fertility treatment.
Pregnancy tests measure the levels of hCG in the blood or urine to indicate the presence or absence of a fertilized egg. In particular, most pregnancy tests employ an antibody that is specific to the β-subunit of hCG (βhCG). This is important so that tests do not make false positives by confusing hCG with LH and FSH. (The latter two are always present at varying levels in the body, while hCG levels are negligible except during pregnancy.) The urine test is a chromatographic immunoassay that can detect levels of βhCG as low as 25-100 mIU/ml. The urine should be the first urine of the morning when hCG levels are highest. If the specific gravity of the urine is above 1.015, the urine should be diluted. The serum test, using 2-4 mL of venous blood, is a radioimmunoassay (RIA) that can detect βhCG levels as low as 5 mIU/ml and allows quantitation of the βhCG concentration. The ability to quantitate the βhCG level is useful in the evaluation of ectopic pregnancy and in monitoring germ cell tumor|germ cell and trophoblastic tumors.
Hydatiform moles ("molar pregnancy") may produce high levels of βhCG, despite the absence of an embryo. This can lead to false positive readings of pregnancy tests.
βhCG is also secreted by a some cancers including teratomas, choriocarcinomas and islet cell tumors. When a patient is suspected of harboring a teratoma (often found in the testis|testes and ovary|ovaries but also in the brain as a dysgerminoma), a physician may consider measuring βhCG. Elevated levels cannot prove the presence of a tumor, and low levels do not rule it out (an exception is in males who do not naturally produce βhCG). Nevertheless, elevated βhCG levels fall after successful treatment (e.g. surgical intervention or chemotherapy), and a recurrence can often be detected by the finding of rising levels.
In the world of performance enhancing drugs, hCG is increasingly used in combination with various Anabolic steroid|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 testicle|testes where it triggers the production and release of testosterone. Without LH, the testes shut down their production of testosterone, causing testicular atrophy ("shrinking testicles"). 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.
GNU Free Documentation License. It uses material from the Wikipedia article "Human chorionic gonadotropin".
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