Some of the things that turn down the natural production of testosterone are estrogen, progesterone, androgens, and of course, testosterone itself. Most likely the newest steroid, primobolan, produces the least detrimental effect to natural testosterone production. 10mg of Oxandrin, (which will not aromatize to estrogen at all), will decrease natural production of testosterone 60% [Int. J. of Obes. Relat. Metab. Disord. 1995; 19.9:614-624]. Finiject, (trenbolone or parabolan) causes about three times more natural testosterone shut down than testosterone cyp. [Environ. Qual. Saf. Suppl. 1976; 5:253-264]. Some say this is because trenbolone exerts progesterone-like effects [Fertil. Steril. 1993; 60.6:1062-1068].

Deca has been considered to also have a progesterone-like effect on male fertility and natural testosterone production. Although deca doesn't aromatize to much estrogen, it will down-regulate the natural production of testosterone more than test. cyp shots [see aforementioned reference].