I am surprised at how often this is recommended by primary care physicians; it is most definitely not a good idea!
Administering testosterone (shots, gels, patches, pills, etc.) will increase the blood levels of testosterone, but strongly inhibit the regulatory hormones FSH and LH, which results in a shutdown of sperm production. The administration of testosterone to men is in fact a very effective form of birth control! (although not ideal for many other reasons)
In many men, this condition can be treated with non-testosterone medications which stimulate the production of leutinizing hormone (LH) (which drives the production of testosterone by the testis) and follicle stimulating hormone (FSH) (which stimulates sperm production or spermatogenesis). The most common medication currently used for this is clomiphene citrate, which blocks the effects of estrogen on its receptors, resulting in an increase in both FSH and LH. This will often result in a significant improvement in testosterone, and is often associated with an improvement in sperm count.
It should be noted that this drug, although commonly used in the treatment of female infertility, is not FDA approved for treatment of low testosterone in men or male infertility, and therefore this represents an off-label use of the drug. Nevertheless it does appear to be effective in many men for this condition, and is widely used by fertility specialists, although studies suggest the improvement in pregnancy rates is modest at best.
Aromatase inhibitors such as Arimidex, which block the conversion of testosterone to estrogen, are also helpful in improving testosterone levels, but their effects on sperm production and fertility are inconsistent and often not very helpful.
It should be emphasized that the presence of low testosterone may be result of a number of clinical conditions, and a thorough evaluation by a specialist should be undertaken prior to considering these drugs.