Human chorionic gonadotropin (hCG) is a hormone that consists of two subunits: alpha and beta. The alpha subunit is common to several hormones (luteinizing hormone - LH, follicle-stimulating hormone - FSH and thyroid-stimulating hormone - TSH), and the beta subunit (beta-hCG) is specific to hCG.
It is beta-hCG that is used for immunometric determination of hCG levels in the blood.