There is no legitimate research showing tribulus to raise testosterone levels.
There is research showing it to cause relaxation of the corpus cavernosum. This would increase erectile function. The researches speculated that it was due to increases in nitric oxide (same mechanism as Viagra).
Interestingly, while nitric oxide increases LH (which would increase libido), it actually has an overall negative effect on testosterone due to suppression at the testicular level.
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