From the publication:

Exogenous testosterone considerably impairs spermatogenesis through suppression of the HPG [hypothalamic–pituitary–gonadal] axis, with decreased levels of follicle-stimulating hormone (FSH), luteinizing hormone and intratesticular testosterone. This effect seems to be most frequent or most profound with the use of intramuscular formulations compared with topical agents. The high number of men in their reproductive years who take testosterone, seems to be partly caused by the misconception that testosterone enhances fertility and a lack of knowledge about its contraceptive effects. This misunderstanding is not limited to patients or non-urologists, as 25% of urologists indicate that they would use testosterone for empirical male infertility treatment.

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These studies demonstrate that, in most cases, azoospermia or severe oligospermia will resolve by 4–12 months after cessation of testosterone or other anabolic steroid use.