|
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 demise 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® 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.
Structure
hCG is a glycoprotein composed of 237 amino acids with a molecular mass of 36.7 kDa. It is heterodimeric, with an α (alpha) 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.
Function
hCG promotes the maintenance of the corpus luteum and causes it to secrete the hormone progesterone. Progesterone enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus.
Because of its similarity to LH and FSH, hCG can also be used clinically to induce ovulation in the ovaries as well as testosterone production in the testes. As the most abundant biological source is women who are presently pregnant, some organisations collect urine from gravidae to extract hCG for use in fertility treatment.
Pregnancy testing
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.)
Hydatiform molas ("mola pregnancy") may produce high levels of βhCG, despite the absence of an embryo. This can lead to false positive readings of pregnancy tests.
Tumor marker
βhCG is occasionally secreted by a group of cancers called teratomas. When a patient is suspected of harboring a teratoma (often found in the testes and 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. 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.
| 
