Human chorionic gonadotrophin
Human chorionic gonadotrophin, (US spelling gonadotropin), generally abbreviated as hCG, is known by many as the "pregnancy hormone", particularly for its role in pregnancy detection. hCG is a substance produced by a newly-operating placenta, in a 10-day conceptus. The hormone is a two-part protein; these parts are called α- and β-chorionic gonadotrophin. α-hCG is also found in other hormones, such as luteinising hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH), while the β-hCG is unique. Of all of these, hCG most resembles the luteinising hormone, and due to this, can be used to artificially induce ovulation in infertile women.
hCG and pregnancy
hCG first rises when pregnancy begins with the completion of implantation in the uterus and remains high until birth; as of 2012, home pregnancy tests can detect elevated hCG levels after as little as 4 weeks of pregnancy, roughly 3 weeks after fertilisation. hCG is also produced by some tumours, and so it can also detect these.
There is some evidence that low levels of hCG in the first trimester correlate to increased risk of spontaneous abortion (a term for early, involuntary miscarriage; don't get your conservative panties in a twist). This is a great example of two logical points: One is correlation-causation fallacy, and the other is not relying on statistics too much. Not important to this article, but a nice illustrative point.
One form of fad diet involves eating as few calories as possible, typically in the range of about 500 kcal/day, as opposed to the 2000+ calories needed by the body daily (or the 1300-1700 fed to Auschwitz inmates). As a safeguard against recurring famines, our ancestors evolved a mechanism in which fat cells would store as much as they can to ensure a "safety cushion" when the times of plenty end and famine rolls in. Due to this mechanism, crash diets like this don't work. It's very dangerous and any weight lost is quickly regained, nearly completely by fat cells. Further, attempts to survive on a 500 kcal/day diet run a substantial risk of accruing major vitamin and mineral deficiencies, as it's nearly impossible to get full trace nutrition on such a restricted diet. (The people who push this garbage probably won't trust Big Pharma with a multivitamin.) Finally, hCG dieting carries with it all the associated risks of constant intravenous injections, like being a heroin addict without all the "fun". One purveyor of this bullshit was an endocrinologist by the name of Albert T. W. Simeons.
Simeons was studying malnourished pregnant women in India and overweight males with pituitary problems, both treated with low amounts of hCG. He then claimed that both lost fat rather than lean tissue. He claimed this was because of some sort of hypothalamic programming to get extra reserves to the fetus. He published this in a book called Pounds and Inches, and whenever somebody was obese, he would recommend hCG and a low carb, low-calorie diet. Once Simeons died, the concept was fished out of the toilet by woo-meister Kevin Trudeau. Although the Journal of the American Medical Association, American Journal of Clinical Nutrition, and other studies have shown hCG is not effective for weight loss, the whole thing became more deeply engrained in the pseudo-medicinal circle.
A shortage of hCG for wooey purposes led to more woo with the addition of homeopathy into the mix for this pseudo-obesity cure. This is funny, as according to homeopathy's "like cures like", hCG would actually be taken for anorexia nervosa, but who ever bothers to read about the field you're in? At least in the United States, hCG for weight loss ended with the FDA's decree that such practices were fraudulent and ineffacious. If only they'll do it for all alternative medicine.
The end of homeopathic hCG has led to the rise of liquid food supplements sold to be taken while following Simeons' 500 kcal/day diet, sold under names such as Activ8 X Drops, Liquid Diet Drops and Secret Diet Drops (the latter sold by an acolyte of Kevin Trudeau). These are a mixture of non-essential amino acids and plant extracts, many of them linked to single studies suggesting they may have an effect on weight loss, if taken in vastly larger doses (i.e., possibly toxic) than in the supplements. Even though they are an entirely different product to hCG, they are still marketed with claims lifted directly from Pounds and Inches.
Perhaps the most pertinent question is: Is the hCG diet safe and effective? The answer, according to the Mayo Clinic, is no to both. No new hCG evidence has been published since the 1995 meta-analysis in the British Journal of Clinical Nutrition, which found that hCG does not suppress appetite or promote weight loss. Obviously this is wrong according to those who make a living selling the hCG diet (history shows that miracle diet peddlers are much more trustworthy than doctors and scientists, after all), so they have set up the "HCG Diet Council" to produce evidence supporting the hCG diet, a classic piece of policy-based evidence making.
hCG is particularly promoted among bodybuilders using the scientifically unsupported argument that it prevents the body from burning lean muscle during very low calorie dieting. Real hCG is touted for use after aggressive steroid cycles to bring hormone levels back to normal and restore the function of the testes. 
- I just want to point out how incredible it is that for ten dollars you can purchase a stick with a state of the art ELISA test that can detect minuscule levels of a single hormone with pinpoint accuracy. Science: it works, bitches.
- Reason for the difference is because "pregnancy" is post hoc determined to have "begun" starting at the last day of the last menstrual cycle before conception.
- "Association between low day 16 hCG and miscarriage after proven cardiac activity." Tong et al., Obstetrics & Gynecology. 2006 Feb;107(2 Pt 1):300-4.
- We don't know if low hCG is caused by a flaw in the pregnancy that leads to abortion, or if the low hCG levels are the cause of miscarriage: all we know is that the two are related. Please leave your wild-assed hypotheses in the talk page.
- It's just a risk factor; many low day-16 hCG pregnancies are carried to term and produce healthy babies.
- Dr. Simeons' hCG Diet Food Choices
- Ineffectiveness of human chorionic gonadotropin in weight reduction: a double-blind study (PDF)
- hCG Worthless as Weight-Loss Aid
- hCG Diet Products are Illegal
- [The Secret Diet Drops ad on Vimeo, Archive copy at the Wayback Machine
- HCG Diet, Mayo Clinic
- [The effect of human chorionic gonadotrophin (HCG) in the treatment of obesity by means of the Simeons therapy: a criteria-based meta-analysis] by Lijesen GK et al. British Journal of Clinical Pharmacology 49:237–243, 1995.