Vitamin D is a steroid hormone synthesized in the skin from sunlight and obtained from diet and supplements. Deficiency is extremely common (estimated 40-50% of the US population) and is associated with low testosterone, depression, fatigue, poor immune function, and increased fracture risk. Multiple studies show a positive correlation between vitamin D status and testosterone levels.
Supplementation with 2,000-5,000 IU daily is generally recommended for deficient individuals.