Is complete HPTA shutdown possible, or a myth?
Conclusion: Fact. After long cycles (12 months), shutdown can occur where normal HPTA is not regained and is not helped by standard PCT protocol. However, a single 100ug dose of Triptorelin reportedly restores function. The latter needs more data (both medical and anecdotal) to be verified.
I found a fantastic research paper here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4854084/
Within the article, it references several other medical cases and studies that show that:
...data also suggest that a longer exposure to exogenous testosterone, Asian ethnicity, and older age may result in a prolonged recovery time after treatment cessation.
The supporting case is a 40 year old man, who after prolonged use of AAS was persistently infertile. It's easy to dismiss this as a fringe case due to age, especially if the reader is in their 20's and 30's. But male infertility at age 40 is not normal. In fact, I found a chart on Quora referencing WHO data here that shows that a man's peak fertility is actually between 40-49 (admittedly, study was done in freaking Gambia of all places). There's some caveats to that, such as offspring for 'older' (and it's not clear what they defined as older) fathers have higher incidence of cancer in their offspring.
Either way, HPTA shutdown is not normal at middle age.
This is absolutely a concern to the average male, and for that reason believe that the conventional wisdom of 8-12 weeks cycled on followed by time off is valid. How much time off is necessary will be a future topic. /u/MezDez mentioned Triptorelin as a potential compound that restores HPTA after shutdown, and at least one study backs this up:
INTERVENTION(S): A single dose (100 μg) of triptorelin (triptorelin test).
MAIN OUTCOME MEASURE(S): Clinical symptoms, androgen normalization, levels of serum testosterone, follicle-stimulating hormone, and luteinizing hormone.
RESULT(S): Within 1 month, the patient's serum testosterone was in the normal range, and he reported a return to normal energy and libido.
Unfortunately, there's not a lot of medical studies on the compound, and anecdotal accounts are sought to help validate these claims.