Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy that keeps the pregnancy viable in its earliest weeks. It’s also the hormone that pregnancy tests detect. But hCG does more than signal a positive test result. It plays roles in fetal development, thyroid function, and fertility medicine, and it serves as a diagnostic marker in certain cancers.
How hCG Supports Early Pregnancy
After a fertilized egg implants in the uterine wall, cells in the developing placenta called syncytiotrophoblasts begin producing hCG. The hormone’s most critical job is keeping the corpus luteum alive. The corpus luteum is a temporary structure on the ovary that forms after ovulation, and it produces progesterone, the hormone that maintains the uterine lining so an embryo can survive. Without hCG, the corpus luteum would break down within about two weeks, progesterone would drop, and the pregnancy would end.
hCG does this by binding to the same receptors that luteinizing hormone (LH) uses on cells in the corpus luteum. This activates a signaling chain that ramps up the enzymes responsible for making progesterone. The corpus luteum continues producing progesterone until roughly weeks 8 to 12 of pregnancy, at which point the placenta takes over hormone production on its own.
hCG Levels During Pregnancy
hCG can appear in blood and urine as early as 10 days after conception. In the first 8 to 10 weeks, levels rise rapidly, nearly doubling every three days. At 3 weeks of pregnancy, a typical range is 5 to 72 mIU/mL. Levels peak around week 10, then gradually decline for the rest of the pregnancy.
This rapid rise is what makes hCG useful for pregnancy testing. Blood tests can detect levels as low as 5 mIU/mL (non-pregnant women typically fall below that threshold). Home urine tests require somewhat higher concentrations, which is why they’re most reliable after a missed period rather than immediately after conception. In healthy men, hCG levels are normally below 2 mIU/mL.
Clinicians also track hCG doubling times in early pregnancy. If levels aren’t rising as expected, it can signal a potential miscarriage or an ectopic pregnancy. Unusually high levels may point to a multiple pregnancy or, more rarely, a molar pregnancy.
Roles Beyond Sustaining the Corpus Luteum
hCG has three main functions during pregnancy: maintaining the corpus luteum, stimulating the fetal testes in male pregnancies, and influencing the mother’s thyroid gland.
In male fetuses, hCG acts on developing Leydig cells in the testes, prompting them to produce testosterone. This testosterone is essential for male sexual differentiation, driving the development of male reproductive anatomy during the first trimester. Without this hormonal signal, male genitalia would not form properly.
hCG also has a mild thyroid-stimulating effect in the mother because its molecular structure partially resembles thyroid-stimulating hormone (TSH). This can slightly boost thyroid hormone output during the first trimester, which is one reason some pregnant women experience symptoms that mimic mild hyperthyroidism early on.
hCG in Fertility Treatments
Because hCG mimics LH, it’s widely used in reproductive medicine to trigger ovulation. In fertility cycles using medications to stimulate egg development, an hCG injection (often called a “trigger shot”) tells the ovaries to release mature eggs at a predictable time. Doses typically range from 5,000 to 10,000 IU, and ovulation generally follows within 36 to 40 hours. This timing allows doctors to schedule insemination or egg retrieval with precision.
In men, hCG is used to treat certain forms of low testosterone. It stimulates the Leydig cells in the testes the same way LH would, but with a longer half-life, meaning it stays active in the body longer. This makes it useful for men who need testosterone support but want to preserve fertility, since standard testosterone replacement can suppress sperm production. hCG keeps the testes active and producing both testosterone and sperm. It’s self-administered as a subcutaneous injection on a regular schedule.
hCG as a Cancer Marker
Outside of pregnancy and fertility treatment, elevated hCG in the blood can be a warning sign. Certain cancers produce hCG, and the hormone is used as a tumor marker to diagnose and monitor them. The most important of these are gestational trophoblastic diseases (cancers arising from placental tissue), testicular germ cell tumors, and ovarian germ cell tumors.
In these cancers, tracking hCG levels over time helps doctors assess whether treatment is working. Rising levels after treatment suggest the cancer may be returning. Tumor cells sometimes also release the individual subunits of hCG (the hormone is made of two protein chains), and measuring these free subunits adds another layer of diagnostic information.
The hCG Diet: What the Evidence Shows
hCG has been marketed as a weight loss aid, sold as drops, pellets, and sprays alongside very low-calorie diets (often 500 calories per day). The claim is that hCG redistributes body fat and suppresses hunger. The FDA has been direct about this: hCG is not approved for weight loss, and over-the-counter hCG products are sold illegally.
The prescription drug label itself states there is “no substantial evidence that it increases weight loss beyond that resulting from caloric restriction, that it causes a more attractive or ‘normal’ distribution of fat, or that it decreases the hunger and discomfort associated with calorie-restricted diets.” Any weight lost on these programs comes from eating 500 calories a day, not from hCG. Diets this restrictive carry real risks, including nutrient deficiencies, gallstones, and muscle loss.