Which hormone is known to act as a carcinogen?

Estrogen is known to act as a carcinogen due to its role in promoting the growth and proliferation of certain types of cells, particularly in hormone-sensitive tissues such as breast and endometrial tissues. Elevated levels of estrogen, especially in postmenopausal women, have been associated with an increased risk of developing cancers in these areas. This is often attributed to estrogen's ability to stimulate cell division, which can lead to mutations and ultimately cancerous transformations if the cell cycle is not properly regulated.

In contrast, the other hormones listed may have various effects on health and metabolism, but they do not have the same direct association with cancer promotion. Insulin primarily plays a role in regulating blood glucose levels, testosterone is crucial for male reproductive health but does not carry the same carcinogenic risk as estrogen, and cortisol is involved in stress response and metabolic processes but is not classified as a carcinogenic hormone either. Thus, estrogen stands out as the hormone with established links to cancer risk through its stimulating effects on cell growth in certain tissues.