Vitamin D deficiency doesn’t always cause symptoms. When it does, some of the symptoms may include: difficulty thinking clearly; bone pain; frequent bone fractures; muscle weakness; soft bones that may result in deformities ; unexplained fatigue; Many people don't develop symptoms until their vitamin D levels get very low or have been low for some time. This can make the condition difficult to diagnose. Several factors have contributed to the rising incidence of vitamin D deficiency. These include: wearing sunscreen (sunscreen blocks the sun’s ability to stimulate vitamin D production)