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.
Structure
hCG is a oligosaccharide glycoprotein composed of 244 amino acids with a molecular mass of 36.7 kDa. Its total dimensions are 75x35x30 angstroms. The α (alpha) subunit is 92 amino acids long and has dimensions 60x25x15 angstroms. It is heterodimeric, with an α (alpha) subunit identical to that of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH) and β (beta) subunit that is unique to hCG. βhCG is encoded by six highly homologous genes which are arranged in tandem and inverted pairs on chromosome 19q13.3 - CGB(1,2,3,5,7,8).
The two subunits create a small hydrophobic core surrounded by a high surface area to volume ratio 2.8 times that of a sphere. The vast majority of the outer amino acids are hydrophilic.
Function
hCG interacts with the LHCG receptor and promotes the maintenance of the corpus luteum during the beginning of pregnancy causing 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. Due to its highly negative charge hCG may repel the immune cells of the mother, protecting the fetus during the first trimester. It has also been suggested that hCG levels are linked to the severity of morning sickness in pregnant women.[1]
Because of its similarity to LH, 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 organizations collect urine from gravidae to extract hCG for use in fertility treatment.
Recreational Use With Steroids
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 ("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.
Taken form here
http://en.wikipedia.org/wiki/Human_chorionic_gonadotropin
