Low testosterone, also called low T or hypogonadism, means your body is not making enough testosterone to keep you feeling and functioning well. Testosterone is made mainly in the testicles, under signals from the pituitary gland in the brain. It plays a part in sex drive, energy, muscle and bone strength, red blood cell production, mood, and how your body stores fat.
Low T is diagnosed when two things line up: symptoms that fit, and blood tests that confirm low levels. One without the other is not enough. Plenty of men have low energy for reasons that have nothing to do with hormones, and some men with lower lab numbers feel fine.
The problem can start in the testicles themselves (primary hypogonadism) or in the signals from the brain that tell them to make testosterone (secondary hypogonadism). Knowing which one you have helps guide what happens next.
Low T is one of the most common reasons men explore hormone replacement therapy, but the first step is always confirming the diagnosis.