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 trophoblast (part of the placenta). Its primary role is to cause progesterone secretion by the corpus luteum, thereby readying the uterine lining for implantation by the fertilized egg.

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.)

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.