Severe deficiency: <12 ng/mL = <30 nmol/L
Deficiency: <20 ng/mL = <50 nmol/L
Insufficient: 20–29 ng/mL = 50–75 nmol/L
Normal: 30–50 ng/mL = 75–125 nmol/LVitamin D levels falling within this normal range prevent clinical manifestations of vitamin D insufficiency as well as vitamin D toxicity. Signs and symptoms
In most cases, vitamin D deficiency is almost asymptomatic. It may only be detected on blood tests but is the cause of some bone diseases and is associated with other conditions:

Complications
Rickets, a childhood disease characterized by impeded growth and deformity of the long bones. The earliest sign of vitamin D deficiency is craniotabes, abnormal softening or thinning of the skull. Osteomalacia, a bone-thinning disorder that occurs exclusively in adults and is characterized by proximal muscle weakness and bone fragility. Women with vitamin D deficiency who have been through multiple pregnancies are at elevated risk of osteomalacia. Osteoporosis, a condition characterized by reduced bone mineral density and increased bone fragility
Increased risk of fracture
Muscle aches, weakness, and twitching (fasciculations), due to reduced blood calcium (hypocalcemia)
Periodontitis, local inflammatory bone loss that can result in tooth loss. Pre-eclampsia:  There has been an association of vitamin D deficiency and women who develop pre-eclampsia in pregnancy. The exact relationship of these conditions is not well understood. Maternal vitamin D deficiency may affect the baby, causing overt bone disease from before birth and impairment of bone quality after birth. Respiratory infections and COVID-19:  Vitamin D deficiency may increase the risk of severe acute respiratory infections and COPD.
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