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Hypoparathyroidism

  • Author: Joseph Michael Gonzalez-Campoy, MD, PhD, FACE; Chief Editor: George T Griffing, MD  more...
 
Updated: Jul 25, 2016
 

Practice Essentials

Hypoparathyroidism is a condition of parathyroid hormone (PTH) deficiency. Primary hypoparathyroidism is a state of inadequate PTH activity. In the absence of adequate PTH activity, the ionized calcium concentration in the extracellular fluid falls below the reference range. Primary hypoparathyroidism, the subject of this article, is a syndrome resulting from iatrogenic causes or one of many rare diseases.

Secondary hypoparathyroidism is a physiologic state in which PTH levels are low in response to a primary process that causes hypercalcemia. The primary processes that lead to hypercalcemia are discussed in other articles (see Hypercalcemia).

Treatment of patients with hypoparathyroidism involves correcting the hypocalcemia by administering calcium and vitamin D.[1] Recombinant human PTH (rhPTH[1-84], Natpara) is commercially available in the United States and is indicated as an adjunct to calcium and vitamin D to control hypocalcemia in patients with hypoparathyroidism.

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Pathophysiology

The ionized calcium concentration in the extracellular fluid (ECF) remains nearly constant, at a level of approximately 1 mM. Ionized calcium in the ECF is in equilibrium with ionized calcium in storage pools such as bone, proteins in the circulation, and within the intracellular fluid. The intracellular fluid concentration of calcium is more than 10,000-fold lower than in the ECF. The maintenance of ionized calcium concentrations in the intracellular and extracellular fluids is highly regulated and modulates the functions of bone, renal tubular cells, clotting factors, adhesion molecules, excitable tissues, and a myriad of intracellular processes.

An extracellular calcium-sensing receptor has been isolated from parathyroid, kidney, and brain cells. The extracellular calcium-sensing receptor is G protein coupled. Mutations in the extracellular calcium-sensing receptor have been demonstrated to result in hypercalcemic or hypocalcemic states. Normally, the extracellular calcium-sensing receptor is extremely sensitive and responds to changes in the ECF calcium ion concentration as small as 2%.

In parathyroid cells, the extracellular calcium-sensing receptor regulates the secretion of PTH. Inactivating mutations of the extracellular calcium-sensing receptor lead to hypercalcemia, as observed in familial hypocalciuric hypercalcemia (heterozygous mutation) and neonatal severe hyperparathyroidism (homozygous mutation). Conversely, activating mutations of the extracellular calcium-sensing receptor lead to hypocalcemia, as observed in some families with autosomal-dominant hypocalcemia.

The intracellular mechanism(s) whereby activation of the extracellular calcium-sensing receptor leads to inhibition of PTH exocytosis is unknown. Because pertussis toxin blocks the inhibition of cyclic adenosine monophosphate (cAMP), but not PTH, in response to a high ECF ionized calcium concentration, cAMP is probably not an important second messenger for the extracellular calcium-sensing receptor. Candidate second messengers include protein kinase C, phospholipase A2, and intracellular calcium.

Conversely, a fall in ECF ionized calcium concentration leads to exocytosis of PTH. PTH has the overall effect of returning the ECF ionized calcium concentration to the reference range by its effects on the kidneys and the skeleton.

PTH activates osteoclasts. Osteoclast activation results in bone resorption and a release of ionized calcium into the ECF. Evidence suggests that small pulse doses of PTH activate osteoblasts, with ensuing bone deposition. The effect of PTH on osteoclasts seems more important than the effect on osteoblasts.

PTH inhibits the proximal tubular transport of phosphate from the lumen to the interstitium. In conditions of primary PTH excess, hypophosphatemia tends to occur. Conversely, in hypoparathyroidism, the phosphate concentration in the plasma is within the reference range or slightly elevated.

PTH has a calcium-retaining effect on the distal tubule. The PTH-mediated calcium reabsorption is independent of any effects on sodium or water reabsorption. This effect of PTH is important in hypoparathyroidism because, in the absence of this distal tubular calcium reabsorption, the kidneys waste calcium. This depletes the ECF ionized calcium and increases the urinary calcium concentration.

PTH stimulates renal 1-alpha-hydroxylase, the enzyme that synthesizes formation of 1,25-dihydroxy vitamin D; 1,25-dihydroxy vitamin D allows for better dietary calcium absorption. Thus, 1,25-dihydroxy vitamin D has a synergistic effect with PTH; both contribute to a rise in the ECF ionized calcium concentration.

In the absence of PTH, bone resorption, phosphaturic effect, renal distal tubular calcium reabsorption, and 1,25-dihydroxy vitamin D–mediated dietary calcium absorption cannot occur. Therefore, the consequence of PTH deficiency is hypocalcemia.

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Epidemiology

Hypoparathyroidism has an estimated prevalence in the United States of 37 per 100,000 person-years. In Denmark, it is estimated to be 22 per 100,000 person-years.[2]

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Contributor Information and Disclosures
Author

Joseph Michael Gonzalez-Campoy, MD, PhD, FACE Medical Director and CEO, Minnesota Center for Obesity, Metabolism, and Endocrinology

Joseph Michael Gonzalez-Campoy, MD, PhD, FACE is a member of the following medical societies: American Association of Clinical Endocrinologists, Association of Clinical Researchers and Educators, Minnesota Medical Association

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Yoram Shenker, MD Chief of Endocrinology Section, Veterans Affairs Medical Center of Madison; Interim Chief, Associate Professor, Department of Internal Medicine, Section of Endocrinology, Diabetes and Metabolism, University of Wisconsin at Madison

Yoram Shenker, MD is a member of the following medical societies: American Heart Association, Central Society for Clinical and Translational Research, Endocrine Society

Disclosure: Nothing to disclose.

Chief Editor

George T Griffing, MD Professor Emeritus of Medicine, St Louis University School of Medicine

George T Griffing, MD is a member of the following medical societies: American Association for the Advancement of Science, International Society for Clinical Densitometry, Southern Society for Clinical Investigation, American College of Medical Practice Executives, American Association for Physician Leadership, American College of Physicians, American Diabetes Association, American Federation for Medical Research, American Heart Association, Central Society for Clinical and Translational Research, Endocrine Society

Disclosure: Nothing to disclose.

Additional Contributors

David S Schade, MD Chief, Division of Endocrinology and Metabolism, Professor, Department of Internal Medicine, University of New Mexico School of Medicine and Health Sciences Center

David S Schade, MD is a member of the following medical societies: American College of Physicians, American Diabetes Association, American Federation for Medical Research, Endocrine Society, New Mexico Medical Society, New York Academy of Sciences, Society for Experimental Biology and Medicine

Disclosure: Nothing to disclose.

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