Human chorionic gonadotropin (HCG) is an LH (luteninizing hormone) analog. That means it's biochemically similar enough to the LH hormone to interact with the same receptors, so it can be used to turn on the testicular machinery, sperm, and testosterone production.
HCG and "Size" Gains
There are stories about HCG increasing ejaculation volume (this is true), and increasing penis size. This is true too, but it may only be the case for those with hypogonadism or "micro-penis." The internet chat boards certainly aren't without their stories of slight enlargement with HCG in normal men. In the two studies I found on micro-penis, gains were three-fourths of an inch in length and girth.
Boosted T With Fewer Side Effects?
HCG is a great option for medically boosting testosterone because, unlike testosterone injections, HCG may actually help the hypothalamus gonadal axis as opposed to suppressing it. It also seems to have less impact on estrogen, prostate mass, and cardiovascular parameters compared to the more traditional TRT (testosterone replacement therapy), while being equal or better than TRT in raising testosterone.
This assessment is evidence-based and taken from a well done study on men aged 45-53 with low T. The study compared HCG against transdermal testosterone and two different injectables.
Many doctors give HCG along with their testosterone therapies to keep the hypothalamus working and the testicles from shrinking. Why would the testicles shrink? Testosterone from an outside source turns off the hypothalamus' secretion of LH, and therefore the testicles stop producing sperm and testosterone. This is why ejaculate volume and testicles can shrink in men taking testosterone. This usually isn't a huge issue if the drug isn't abused, but HCG helps keep this from happening.
As an aside, steroids do not shrink the size of the glans penis (the shaft), just the testicles, and only if used in very high amounts for too long.
Using HCG alone is a reliable promoter of testosterone, and may be the safer, more natural option to start with in those with HPG issues. It may also be the best approach for those who've been on testosterone for a long period of time.
Doctor Prescribed HCG Therapies
Based on the studies, there are a few approaches here. If using TRT, then the approach recommended is 250IU of HCG taken as an intramuscular injection (IM) daily. If you're using HCG alone, according to the study above where it was directly compared to TRT, the dose is 2000IU per week.
Most doctors don't like giving such a high dose of HCG all at once for fear of excess estrogen production and desensitization of LH receptors. Although this study didn't show that, it may be a consideration.
Keeping a once daily dose to 500IU or less seems wise, which means you'd be injecting 500IU 1-4 times per week (500IU – 2000IU) for HCG monotherapy.
For more info, see the related links below.