Copy of write-up I found:
Human Chorionic Gonadotropin (hCG) is a peptide hormone that mimics the action of luteinizing hormone (LH). LH is the hormone that stimulates the testes to increase testosterone levels. (1) More specifically LH is the primary signal sent from the pituitary to the testes, which stimulates the leydig cells within the testes to produce testosterone.
When steroids are administered, LH levels rapidly decline. The absence of an LH signal from the pituitary causes the testes to stop producing testosterone, which causes rapid onset of testicular degeneration. The testicular degeneration begins with a reduction of leydig cell volume, and is then followed by rapid reductions in intra-testicular testosterone (ITT), peroxisomes, and Insulin-like factor 3 (INSL3) - All important bio-markers and factors for proper testicular function and testosterone production. (2-6,19) However, this degeneration can be prevented by a small maintenance dose of hCG ran throughout the cycle. Unfortunately, most steroid users have been engrained to believe that hCG should be used after a cycle, during Post-Cycle-Therapy. Upon reviewing the science and basic endocrinology you will see that a faster and more complete recovery is possible if hCG is ran during a cycle.
Firstly, we must understand the clinical history of hCG to understand its purpose and its most efficient application. Many popular “steroid profiles” advocate using hCG at a dose of 2500-5000iu once or twice a week. These were the kind of dosages used in the historical (1960's) hCG studies for hypogonadal men who had reduced testicular sensitivity due to prolonged LH deficiency. (21,22) A prolonged LH deficiency causes the testes to desensitize, requiring a higher hCG dose for ample stimulation. In men with normal LH levels and normal testicular sensitivity, the maximum increase of testosterone is seen from a dose of only 250iu, with minimal increases obtained from 500iu or even 5000iu. (2,11) (It appears the testes maximum secretion of testosterone is about 140% above their normal capacity.) (12-18) If you have allowed your testes to desensitize over the length of a typical steroid cycle, (8-16 weeks) then you would require a higher dose to elicit a response in an attempt to restore normal testicular size and function - but there is cost to this, and a high probability that you won't regain full testicular function.
One term that is critical to understand is testosterone secretion capacity which is synonymous to testicular sensitivity. This is the amount of testosterone your testes can produce from any given level of LH or hCG stimulation. Therefore, if you have reduced testosterone secretion capacity (reduced testicular sensitivity), it will take more LH or hCG stimulation to produce the same result as if you had normal testosterone secretion capacity. If you reduce your testosterone secretion capacity too much, then no amount of LH or hCG stimulation will trigger natural testosterone production - and this leads to permanently reduced testosterone production. (recovering full testosterone production is a topic for another article)
To get an idea of how quickly you can reduce your testosterone secretion capacity from your average steroid cycle, consider this: LH levels are rapidly decreased by the 2nd day of steroid administration. (2,9,10) By shutting down the LH signal and allowing the testis to be non-functional over a 12-16 week period, leydig cell volume decreases 90%, ITT decreases 94%, INSL3 decreases 95%, while the capacity to secrete testosterone decreases as much as 98%. (2-6)
Note: visually analyzing testes size is a poor method of judging your actual testicular function, since testicular size is not directly related to the ability to secrete testosterone. (4) This is because the leydig cells, which are the primary sites of testosterone secretion, only make up about 10% of the total testicular volume. Therefore, when the testes may only appear 5-10% smaller, the testes ability to secrete testosterone upon LH or hCG stimulation can actually be significantly reduced to 98% of their normal production. (3-5) So do not judge how "shutdown" you are by testicular size!
The decreased testosterone secretion capacity caused by steroid use was well demonstrated in a study on power athletes who used steroids for 16 weeks, and were then administered 4500iu hCG post cycle. It was found that the steroid users were about 20 times less responsive to hCG, when compared to normal men who did not use steroids. (8) In other words, their testosterone secretion capacity was dramatically reduced because they did not receive an LH signal for 16 weeks. The testes essentially became desensitized and crippled. Case studies with steroid using patients show that aggressive long-term treatment with hCG at dosages as high as 10,000iu E3D for 12 weeks were unable to return full testicular size. (7) Another study with men using low dose steroids for 6 weeks showed unsuccessful return of Insulin-like factor-3 (INSL3) concentration in the testes upon 5000iu/wk of HCG treatment for 12 weeks (6) (INSL3 is an important biomarker for testosterone production potential and sperm production) 20
In light of the above evidence, it becomes obvious that we must take preventative measures to avoid this testicular degeneration. We must protect our testicular sensitivity. Besides, with hCG being so readily available, and such a painless shot, it makes you wonder why anyone wouldn't use it on cycle.
Based on studies with normal men using steroids, 100iu HCG administered everyday was enough to preserve full testicular function and ITT levels, without causing desensitization typically associated with higher doses of hCG. (2) It is important that low-dose hCG is started before testicular sensitivity is reduced, which appears to rapidly manifest within the first 2-3 weeks of steroid use. Also, it's important to discontinue the hCG before you start Post-Cycle-Therapy so your leydig cells are given a chance to re-sensitize to your body's own LH production. (To help further enhance testicular sensitivity, the dietary supplement Toco-8 may be used)
Based off the above information, an optimal dose of hCG during the cycle would be 250iu every 4 days, or as a less desirable alternative, once a week shot of 500iu. Keep in mind, that the half-life of hCG is 3-4 days, while the half-life of LH is only 1-2 hours. Considering this difference in excretion time, it is best to space each dose of hCG at least 4 days apart for the optimal "peak and valley" replication. However, going more than 7 days between each hCG shot may promote increase the rate of desensitization from lack of LH or hCG stimulation.
If you are starting hCG late in the cycle, one could calculate a rough estimate for their required hCG "kick starting" dosage by multiplying 40iu x days of LH absence. (ie. 40iu x 60 days = 2400iu HCG dose) Remember, since the testes will be desensitized later in a cycle, you will require a higher dose. Also, the maximum daily dose of hCG should not exceed 5000iu, and 4-7 days must be taken off between each shot. Generally, a higher dose will require a longer off period between each shot. (eg., 2500iu = 7 days between each shot)
Note: If following the on cycle hCG protocol, hCG should NOT be used for PCT.
Recap -
For preservation of testicular sensitivity, use 250iu every 4 day starting 14 days after your first AAS dose. At the end of the cycle, drop the hCG two weeks before the AAS clear the system. For example, you would drop hCG about the same time as your last Testosterone Enanthate shot. Or, if you are ending the cycle with orals, you would drop the hCG about 10 days before your last oral dose. This will allow for a sudden and even clearance in hormone levels. This will initiate a strong LH and FSH surge from the pituitary, to begin stimulating your testes to produce testosterone. Remember, recovery doesn't begin until you are off hCG since your body will not release its own LH until the hCG has cleared the system.
In conclusion, we have learned that utilizing hCG during a steroid cycle will significantly prevent testicular degeneration. This helps create a seamless transition from “on cycle” to “off cycle” thus avoiding the post cycle crash
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