If your growth plates are closed, no amount of GH will make you grow. You will probably still get the usual GH side effects though.
See an endocrinologist and have him X-ray your wrist, he will be able to tell you whenever GH is still an option.
Exercising (mainly Squats and Deadlifts) and use of steroids can cause the premature stop of one's growth. As you can imagine there has not been any studies done to prove those statements.
There is no doubt that HGH works, a distant cousin of mine was short, luckily his parents were able to afford HGH treatment. I believe he grew something like 4 inches in the first 6 months.
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