Endocrinology
Hypocalcemia
Hypoparathyroidism (impaired synthesis [eg, 22q deletion syndrome] or secretion);

Vitamin D deficiency (reduced intake or production);

Maternal hyperparathyroidism (fetal hypercalcemia leads to infant suppression of PTH);

Ill neonate;

Hypomagnesemia (causes resistance to PTH); and

High phosphate intake (antagonizes PTH).