Pathogenesis and management of hypoparathyroidism and other hypocalcemic disorders

Hypocalcemia frequently presents as an acute medical emergency or a chronic disorder which is difficult to control. Occasionally, it is found in routine blood screening tests when it is not anticipated. Recent developments in basic endocrine science have contributed greatly to our understanding and treatment of hypocalcemic disorders. The maintanance of a normal serum calcium concentration depends on the balanced actions of parathyroid hormone (PTH), vitamin D, and, to a lesser extent, calcitonin, Recent work on PTH secretion has defined the factors controlling its secretion in normal and abnormal states. In primary hypoparathyroidism, hormone secretion is decreased or absent, while in most other forms of hypocalcemia, secretion is stimulated secondarily by the hypocalcemia. However, acute or chronci disorders associated with hypomagnesemia may also decrease effective PTH secretion. Patients with the rare disorder, pseudohypoparathyroidism, have defects of hormone action and usually have elevated levels of PTH prior to therapy. Several forms of pseudohypoparathyroidism have been recognized, each representing a defect as a different site of PTH action. Calcitonin excess, as noted in medullary carcinoma of the thyroid, could theorectically cause hypocalcemia, but rarely does so. Vitamin D undergoes a series of two carefully controlled hydroxylation reactions leading to the final active metabolite, 1,25-hihyroxycholecalciferol. Chronic ingestion of certain drugs can lead to osteomalacia and hypocalcemia by potentiating the metabolism of vitamin D to inactive compounds. At least one form of rickets has been shown to result from a specific enzyme defect in the vitamin D pathway. Severe renal damage limits the conversion of vitamin D to its active form and contributes to vitamin D resistance. Current progress in the area depends on the development of procedures for the measurement of the metabolites in plasma and assessing the role of the vitamin (hormone) in normal and abnormal physiology. Although the therapy of acute hypocalcemia is usually readily accomplished, chronic hypocalcemia remains a very difficult therapeutic problem. Vitamin D, the hallmark of therapy, is a long-acting drug with a narrow therapeutic range. The complications of the disease and therapy are sometimes irreversible. The unraveling of vitamin D metabolism has led to the development of new therapeutic agents which might provide better relief of chronic hypocalcemic states. This review related new information about calcium homeostasis to the clinical situation encountered in the patient with hypocalcemia.