Structural Biochemistry/Human Chorionic Gonadotropins

Introduction
Human Chorionic Gonadotropin (hCG) is a glycoprotein hormone made up of an alpha and beta subunit. The alpha subunit making up hCG is the same one that makes up the alpha subunits of the luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone. While the beta subunit is comparable with the beta subunit of the luteinizing hormone, there are differences that make the beta subunit of hCG unoique. However because of the similarities between hCG and luteinizing hormone, hCG is viewed as a much more active version of luteinizing hormone. Both hormones are recognized by the luteinizing hormone/choriogonadotropin receptor (LHCGR), which is located in the ovaries, uterus, and breasts. It has been dubbed as the pregnancy hormone because of its multiple functions which affect the pregnancy process. It is produced in the placenta of a pregnant woman by the syncytiotrophoblast cells, which are the outer cells of the blastocyst.

Function
The functions of the hCG allow for a safe pregnancy to take place. It gets activated by the LHCGR and stimulates the production of the steroid hormone, progesterone. Progesterone allows for the lining of the uterus to thicken so that a fertilized egg will be able to stick and develop into a fetus. hCG will also bind to specific regions on the corpus luteum, which is in the ovaries, which allows for the production of progesterone to continue. If hCG did not bind the corpus luteum, the amount of progesterone in the body would drop and the thickened uterus lining would be released during the woman's period. Overall, hCG prevents a woman from getting her period when she is conceived with a child.

Pregnancy Tests
Since hCG is only produced in pregnant women, the presence of hCG can be used to test for pregnancy. Traces of hCG can be found in urine as early as 11 days after fertilization and found in blood after about two weeks. All pregnancy test use immunoassays, which determine the concentration of hCG in a woman. In the mid-1900s when immunoassays were being developed, there were complications in increasing its sensitivity and being able to recognize hCG and not luteinizing hormone. Because of the structural differences in the beta subunits of hCG and luteinizing hormone, immunoassays that could differentiate the two hormones were developed. The pregnancy test determines the milli-international units of hCG in one mL of blood or urine. If there is less than 5 milli-international units in 1 mL, then the test comes out negative. A concentration of 25 milli-international units in 1 mL is a positive test. During the first two to three months of pregnancy, the hCG levels should increase dramatically and then decrease until about six months of pregnancy, when the level should remain relatively consistent. When the hCG level is lower than the expected amount, it serves as an indication of a miscarriage that may happen or has already happened, or a ectopic pregnancy, which means the fetus is developing outside of the uterus and will be fatal to the fetus, but may be fatal to the mother as well. If hCG levels are too high, it is likely to either be a multiple pregnancy or a molar pregnancy, which is when the cells turn into abnormal tissue, rather than a fetus, but the body behaves as if an actual pregnancy has taken place. Molar pregnancies are characterized by high levels of beta-hCG. After giving birth or a miscarriage, the hCG levels should return to normal (which is below 5 milli-international units per mL) within two months.

Genetic Disorders Testing
The Triple Screen Test is conducted on women during their fourth to sixth months of pregnancy to determine the probabilities of the fetus carrying certain genetic disorders. This test is not very reliable but is recommended for women who are over the age of 35, have a family history of genetic disorders, or have been sick at any time during their pregnancy. The test is determines the levels of hCG and two other substances, estriol and alpha-fetoprotein in the mother's blood.

Role As a Diet Supplement
A significant level of hCG prevents women from feeling hungry as their standard amount and also encourages the body to use fat, rather than carbohydrates, for energy. Some people who want to lose weight have been looking for hCG injections or oral supplements, in hopes of an easy diet. Because their hunger would be suppressed, they would be able to easily limit themselves to about 500 calories daily, without slowing down their metabolism. However, this is dangerous because people would not be able to get the necessary amount of nutrition, which can lead to dangerous side effects, so supplemental hCG is not FDA approved and discouraged by medical professionals.