Serum Human Chorionic Gonadotropin (HCG)
Hormonal Structure, Function, and Clinical Significance
INTRODUCTION
Serum human chorionic gonadotropin (HCG) is a glycoprotein hormone produced primarily during pregnancy. It is secreted by trophoblastic cells of the placenta , especially by the syncytiotrophoblast , which develops after a fertilized egg implants into the uterine lining.
Structurally, HCG is a dimeric glycoprotein with a molecular weight of about 36,700 . It is composed of two subunits:
α-subunit (alpha):
This part of the molecule is structurally very similar to the alpha-subunits of other pituitary hormones such as FSH (follicle-stimulating hormone) , LH (luteinizing hormone) , and TSH (thyroid-stimulating hormone) . Because of this similarity, some cross-reactivity between these hormones can occur in testing.
β-subunit (beta):
This is the unique part of the hormone that distinguishes HCG from FSH, LH, and TSH. The β-subunit's unique structure allows laboratory tests to specifically identify HCG in the blood or urine.
Once pregnancy begins, the developing placenta produces increasingly large amounts of HCG. After entering the mother's bloodstream, the hormone can be detected in both serum (blood) and urine , which is why HCG forms the basis of most pregnancy tests. Its presence serves as an early marker of pregnancy and plays an important role in supporting the developing embryo.
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