Hypocalcemia occurs when the total serum calcium concentration is less than 7.0 mg/dL and the ionized calcium is less than 3.5 mg/dL (depending on the particular ion‐selective electrode used). There are many causes of hypocalcemia, these include;
Vitamin D deficiency

Chronic renal failure

Magnesium deficiency

Alcoholism

Biphosphonate therapy - drugs used to treat high blood calcium levels or pills used to treat osteoporosis.
 
Certain types of leukemia or blood disorders

A complication of chemotherapy, tumor lysis syndrome, occurs when your body breaks down tumor cells rapidly, after chemotherapy.  This may cause hypocalcemia, high blood potassium levels, and other electrolyte abnormalities. This is very serious, and if your blood test results indicate you suffer from it, your doctor or health care provider will need to closely monitor you during this time.

Drugs such as diuretics, estrogens replacement therapy, fluorides, glucose, insulin, excessive laxative use, and magnesium may also lead to hypocalcemia.

Certain things in your diet, like caffeine, phosphates (found in soda pop), and certain antibiotics may make it difficult for you to absorb calcium.

Vitamin D, however, helps you to absorb calcium in your body.