Human Chorionic Gonadotropin (hCG) is a sialoglycoprotein hormone secreted by the trophoblastic cells of the placenta during pregnancy. Its production increases shortly after implantation of the fertilized ovum in the uterine wall. Although its role in the female reproductive cycle is not clear, hCG is instrumental in the maintenance of the corpus luteum at the beginning of the gestation period.
hCG is a heterodimer, which can be dissociated to yield two dissimilar subunits: α-hCG and β-hCG. The α-subunit (approximate molecular weight of 14,500 Daltons) is virtually identical to the α-subunits of the related pituitary hormones Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH), and Thyroid Stimulating Hormone (TSH). The biological activity of hCG is dependent on the distinct make-up of the β-subunit (approximate molecular weight of 22,200 Daltons), which differs in amino acid sequence from the β-subunits of FSH, LH, and TSH.
Immunoassays for hCG serum levels are useful in the detection and/or verification of normal pregnancy, as elevated levels of hCG are reportedly detectable as early as seven days after conception. In addition, low levels of serum hCG may help diagnose ectopic pregnancy, while elevated levels of serum hCG have been reported in patients with trophoblastic disease, choriocarcinoma, and various other types of cancer.