Typically, serum and urine concentrations of hCG rise exponentially in the first trimester of pregnancy, doubling about every 24 hours during the first 8 weeks.
Levels of hCG can vary widely between women with normal pregnancies. HCG is an important hormone in pregnancy, and its clinical utility is primarily centered around its detection in early pregnancy, along with serial measurement during pregnancy and pregnancy-related complications. Urine testing, however, is more convenient, affordable, comfortable for patients, has a fast turnaround (5 to 10 minutes), and does not require a medical prescription. Serum testing is much more sensitive and specific than urine testing. Many over-the-counter urine pregnancy tests do not detect hyperglycosylated hCG, which accounts for most of the hCG in early pregnancy, resulting in a wide range of sensitivities of these tests.
Urine assays are similar, although many detect total hCG levels greater than 20 mIU/mL. There are more than 100 different assays commercially available which results in significant variability in reported values. Assays involve washing away the excess serum components and measuring the amount of remaining labeled hCG to give a quantitative result. This means that they use 2two antibodies that bind to the hCG molecule, a fixed antibody and a radiolabeled antibody which adhere to different sites on the molecule, sandwiching and immobilize the molecule to make it detectable. Serum tests for hCG are immunometric assays. The beta subunit is degraded in the kidney to make a core fragment which is measured by urine hCG tests. HCG is primarily catabolized by the liver, although about 20% is excreted in the urine. There are multiple forms found in the serum and urine during pregnancy including the intact hormone and each of the free subunits. The hormone itself is a glycoprotein composed of two subunits, the alpha and beta subunits. Trophoblastic cancers (hydatidiform mole, choriocarcinoma, and germ cell tumors) are associated with high serum levels of hCG-related molecules. As previously mentioned, certain malignancies can also produce either hCG or hCG-related hormone. Smaller amounts of hCG are also produced in the pituitary gland, the liver, and the colon. The hormone stimulates the corpus luteum to produce progesterone to maintain the pregnancy. Human chorionic gonadotropin is a hormone produced primarily by syncytiotrophoblastic cells of the placenta during pregnancy.
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