Like I explained in this video, HCG increases upstream hormones shut down by TRT.
https://www.excelmale.com/content.php?259-Why-is-HCG-Important-for-Men-on-Testosterone-Replacement-Therapy
17-OH-progesterone is one of those upstream hormones. Higher levels have been linked to improved efficacy of HCG on testicular function. The old studies used to aspirate testicular tissue to determine intratesticular T as a way to measure HCG efficacy. Of course, that is risky and uncomfortable. Testing for 17-OH-progesterone blood levels may be as effective and easier in determining HCG's efficacy. The study also enforces my view that HCG doses of 350-500 IU are needed for proper testicular function. I do not believe in daily low dose HCG as recommended by Dr Crisler and others.