Testosterone. Get exam and proper studies. If low there are gels (better than injections b/c daily gel gives you a steady daily level. Shot gives a burst initially but it can wear out in a few weeks). Need to check lipid, hemoglobin, liver test, testosterone and prostate level every 6 month if taking testosterone.
Treat the cause. It's important to emphasize dr. Irizarry's point that before you treat with gel, cream, foam, patch, injection, etc, you need to figure out why your testosterone is low. Don't jump directly to treating low testosterone because you could miss a very treatable cause, i.E pituitary tumor, drug side effects...
Very carefully! After a thorough exam to exclude other treatable causes of low testosterone, you need to ask if future fertility is an issue. Testosterone replacement of any sort can render you infertile. If you want (more) children, other options are available to increase testosterone while maintaining sperm production. Btw, testosterone shots should probably taken at least weekly to prevent rollercoaster effect.
Many causes! Do you know chronic opiod therapy causes hypogonadism and low-t. Pain clinics create low-t males. Obesity causes low-t, . Because testosterone is converted into estradiol-therfoe losing weight helps. My own published research in 2007 shows that supplemental dhea (100mg/day) and zinc (50mg) can raise testosterone nearly 80% (free-t, not total-t, which is what counts).