U.S. beef is either injected with or fed testosterone, progesterone, trenbolone acetate, 17 beta-estradiol, zeranol, and MGA

• 17 beta-estradiol: 17 beta-estradiol is a synthetic form of progesterone shares similar characteristics with a class of other molecules called endocrine disruptors that imitate other human hormones in the body. It is s a complete carcinogen. -- Scientific Committee on Veterinary Measures Relating to Public Health
• Zeranol:  Zeranol is a progesterone-imitator which has been linked to cancer in a recent study. -- Young C. Lin, DVM, PhD, Ohio State University 
• Trenbolone: Trenbolone is a synthetic anabolic steroid that has produced pancreatic tumours, liver tumors and hyperplasia.
• Progesterone: Progesterone is considered a probable carcinogen by the National Toxicology Program at the National Institutes of Health
• MGA: Melengestrol acetate increased incidence in mammary tumours in laboratory studies.  It is considered a suspected carcinogen by the USDA.

Side Effects

It is "very likely" that hormone residues in North American beef is a factor in the early onset of puberty among girls in recent decades. -- Carlos Sonnenschein, Professor of Anatomy and Cellular Biology, Sackler School of Graduate Biomedical Sciences, Tufts University.