Recently, an anonymous user posted what he thought was a funny story to the popular social news site Reddit, describing how his male friend took a pregnancy test – which came back positive. After many commenters urged the man’s friend to go the doctor, it turned out the friend actually had testicular cancer.

What a pregnancy test measures is a hormone called human chorionic gonadotropin (hCG). This hormone is manufactured naturally in pregnancy by the placenta and later by the syncytiotrophoblast cells. The pregnancy test specifically measures the beta subunit of hCG to predict the presence of a pregnancy and if it is developing normally.

However, there are some cancer cells that also produce beta hCG. These cancers include some that are particular to males like testicular cancer, but also ones that are related to the female reproductive system. There are certain cancers such as choriocarcinoma, ovarian germ cell tumors and molar pregnancies -- which are masses inside the uterus -- that have been linked with high elevations of hCG.

While a routine five-week pregnancy could have a range of beta hCG in the 7,000s mIU/ml range, for example, in some of these cancers, the range could be elevated significantly above the range expected for a normal pregnancy. In males, you should never find any levels of hCG, which is why this man’s positive test was so irregular – and lead to the discovery of his tumor.

After hearing this story, some people have asked me how I feel about the hCG diet, and I keep saying that I certainly would not recommend it.