I found this on a search. Nelson Montana is interviewing scientist Victor Conte founder of Balco Laboratories:
VC: Exactly. However, we found a number of interesting things concerning serum total testosterone levels. One pro bodybuilder had a level of 21,000 ng/dL! Another bodybuilder had a level so low it couldn't be detected. The average testosterone levels of the 24 pro bodybuilders was about 3,500 ng/dL. We discovered that the bodybuilder with the undetectable level had been using Halotestin. This anabolic steroid should be avoided because it completely shuts down the production of endogenous testosterone and has long lasting and enduring negative effects upon the endocrine system.
Another interesting finding among the 24 pro bodybuilders is that 16 of them were also taking growth hormone. Eight of these 16 were also taking Nolvadex. What we discovered is that Nolvadex reduces the effect of the growth hormone—as measured by IGF-1 levels—by about 50%.
The reason why this is counterproductive is because Nolvadex increases IGF binding protein (BP3) concentrations which in turn decreases the activity of circulating IGF-1. Testosterone is to SHBG as IGF-1 is to IGF-BP3.