What are worst effects of a vitamin d deficiency?

Rickets: In children severe vitamin d deficiency can cause impaired growth, bowing of lower extremities, bone and muscle pain. In adults it may contribute to developing osteoporosis. Muscle weakness may occur. More controversial is a deleterious effect on diabetes, various cancers, infections and a variety of other problems.

Calcium deficiency : The worst consequence of vitamin d deficiency is low blood calcium. Vitamin d is needed to absorb calcium from the gut. Severe cases of vitamin d deficiency can therefore be life threatening if blood calcium levels drop low enough.