Some common signs and symptoms of low T include low sex drive, erectile dysfunction, depleted energy/fatigue, decreased strength and endurance, loss of body/facial hair, decreased bone mass, mood changes like depression, difficulty concentrating, weight gain with muscle loss, insomnia, and hot flashes. Testing blood levels is the only way to diagnose a testosterone deficiency definitively. Most endocrinologists consider total testosterone levels below 300 ng/dL to confirm low T diagnosis.