The parathyroid glands are small, rice-sized glands of the endocrine system located in the lower neck. They produce the parathyroid hormone and regulate blood levels of calcium and phosphorus, blood clotting and muscle contractions (neuromuscular excitation). If abnormal levels of blood calcium are detected, further testing may be conducted. Low levels of calcium in the blood could be caused by secondary hyperparathyroidism. Conditions that lead to hyperparathyroidism and are associated with lower levels of calcium include: - Severe renal disease - Gastric and intestinal weight loss surgery - Celiac disease - Crohn's disease - Vitamin D deficiency Contrarily, high levels of calcium can be caused by primary hyperparathyroidism.