Free beta chorionic gonadotropin measurement in serum is used for:
- Diagnosis and monitoring of trophoblastic diseases (molar pregnancy, choriocarcinoma)
- Diagnosis and monitoring of testicular, hCG-secreting, tumors (in some cases the tumor secretes only free beta-hCG)
- Early monitoring of in vitro fertilization results
- Determination of free beta-hCG in maternal serum in combination with PAPP-A (pregnancy-associated plasma protein) is useful for prenatal screening for Down syndrome, trisomy 18, and other fetal chromosomal abnormalities, in the first trimester of pregnancy (double test)
- Determination of free beta-hCG in maternal serum in combination with AFP and unconjugated estriol is useful for prenatal screening for Down syndrome, trisomy 18, neural tube defects, and other disorders, in the second trimester of pregnancy (triple test)
Human chorionic gonadotropin (hCG) Is a glycoprotein hormone that is normally found in the blood and urine only during pregnancy. It is synthesized by the placenta (trophoblastic tissue) and prevents corpus luteum destruction in the first weeks of pregnancy.
Intact hCG consists of 2 subunits (alpha and beta subunits) that are linked by a noncovalent bond. These subunits can also be represented in a free, unbound form. The alpha subunit has an identical structure to the homologous subunit of pituitary glycoprotein hormones (LH, FSH, and TSH). Beta subunits are specific for each of these hormones, which determines their specific biological activity.
The serum of pregnant women mainly contains intact hCG. However, a minor fraction of alpha- and beta‑subunits circulate in an unbound form. hCG-free beta subunit fraction represents about 1% of total intact hCG and is biologically inactive. Studies have shown that the level of the free fraction of the beta-subunit of hCG is the most accurate marker of the presence of fetal aneuploidy in the first trimester of pregnancy.
Possible Interpretations of Pathological Values
Increase: Trisomy 21 (Down syndrome), Shereshevsky-Turner syndrome, multiple pregnancies, chorionic carcinoma, gestational diabetes. Medications: synthetic progestogens.
Decrease: Trisomy 13 (Pataus syndrome), trisomy 18 (Edwards syndrome), anencephaly, ectopic pregnancy, pregnancy at risk, intrauterine fetal demise, preeclampsia.