I have the study.
"Anabolic" effects of methandienone in men undergoing athletic training
G.R Hervey October 1976, Published in The Lancet
100 mg Dbol per diem for 6 weeks. LH, FSH had no statistically significant decline, but testosterone fell by about half. That suggests that there is primary transient hypogonadism because there is enough LH, FSH in the blood but the testicles are not producing enough testosterone. That leads me to think that there is some negative feedback directly in the testicles.
It could also be that there is excessive conversion to estrogen, but I think the testicles are also being inhibited.