As shown in this article, what happens is that the 5ar enzyme starts to saturate at higher doses of testosterone. DHT levels still increase with the higher doses, but less for each incremental increase in testosterone. What's interesting to consider is that there may be some competitive inhibition of other 5ar processes at higher T levels, leading to problems.... In fact if injecting more Testosterone would "burn 5 alpha reductase" we would see a drop in DHT as well, but not quite I believe.
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