Medical School · Year 2 · Endocrine · includes a quiz and discussion video

Lecture 11: Parathyroid Disorders

Unit 2.3: Endocrine System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the clinical features and causes of hypercalcemia
2. Explain primary hyperparathyroidism and its management
3. Describe secondary and tertiary hyperparathyroidism
4. Explain the clinical features and causes of hypocalcemia
5. Describe hypoparathyroidism and pseudohypoparathyroidism
6. Explain the diagnostic workup and treatment of calcium disorders

Lecture Outline

I. Hypercalcemia - Overview

Hypercalcemia is among the most commonly encountered electrolyte abnormalities in clinical practice, and its severity classification guides the urgency of evaluation and treatment. Mild hypercalcemia is defined as a total calcium of 10.5-12 mg/dL (ionized calcium 5.6-8 mg/dL), moderate hypercalcemia as 12-14 mg/dL (ionized calcium 8-10 mg/dL), and severe hypercalcemia as greater than 14 mg/dL (ionized calcium greater than 10 mg/dL). Severe hypercalcemia constitutes a medical emergency requiring immediate intervention, whereas mild elevations may be asymptomatic and discovered incidentally on routine blood work.

The differential diagnosis of hypercalcemia encompasses a broad range of conditions that can be organized by pathophysiologic mechanism. PTH-mediated causes include primary hyperparathyroidism, familial hypocalciuric hypercalcemia (FHH), and tertiary hyperparathyroidism. Malignancy causes hypercalcemia through several mechanisms: parathyroid hormone-related peptide (PTHrP) secretion, lytic bone lesions, and ectopic production of 1,25(OH)2D. Vitamin D-related causes include exogenous intoxication and endogenous production in granulomatous disease. Medications implicated in hypercalcemia include thiazide diuretics, lithium, and vitamin A. Endocrine disorders such as thyrotoxicosis and adrenal insufficiency may elevate calcium. Other causes include immobilization and milk-alkali syndrome. A useful mnemonic for remembering this differential is "CHIMPANZEES": Calcium excess (milk-alkali, supplements), Hyperparathyroidism, Iatrogenic (thiazides, lithium), Malignancy, Paget disease, Addison disease, Neoplasm, Zollinger-Ellison (MEN1), Excess vitamin D or A, Endocrine (thyrotoxicosis), and Sarcoidosis.

The most common cause of hypercalcemia differs dramatically based on the clinical setting, and recognizing this distinction is essential for efficient workup. In the outpatient setting, primary hyperparathyroidism accounts for approximately 90% of cases, typically presenting as an incidental finding on routine laboratory testing. In the inpatient setting, malignancy is the dominant cause, responsible for approximately 65% of cases, and generally indicates advanced disease with a poorer prognosis. This epidemiologic distinction should immediately inform the clinician's diagnostic approach when encountering an elevated calcium level.

<image>Panel A: Primary classification of hypercalcemia causes dividing PTH-mediated (primary hyperparathyroidism, FHH, tertiary HPT) from non-PTH-mediated (malignancy, vitamin D excess, medications, endocrine disorders). Panel B: Common causes by clinical setting showing primary hyperparathyroidism as the most common outpatient cause (approximately 90%) and malignancy as the most common inpatient cause (approximately 65%). Panel C: CHIMPANZEES mnemonic listing calcium excess, hyperparathyroidism, iatrogenic, malignancy, Paget disease, Addison disease, neoplasm, Zollinger-Ellison, excess vitamin D or A, endocrine, and sarcoidosis. Panel D: Severity classification table showing mild (10.5-12 mg/dL), moderate (12-14 mg/dL), and severe (greater than 14 mg/dL) total calcium with corresponding ionized calcium ranges.</image>

II. Hypercalcemia - Clinical Features

The clinical manifestations of hypercalcemia are classically summarized by the mnemonic "Stones, Bones, Groans, Moans," which captures the four major domains of involvement. "Stones" refers to nephrolithiasis and nephrocalcinosis resulting from chronic calcium elevation. "Bones" encompasses bone pain, pathologic fractures, and osteitis fibrosa cystica, the classic skeletal lesion of hyperparathyroidism. "Groans" describes the gastrointestinal complaints including abdominal pain, constipation, pancreatitis, and peptic ulcer disease. "Moans" refers to the psychiatric and neurologic manifestations such as depression and confusion. The severity and breadth of symptoms correlate with both the degree and rapidity of calcium elevation.

Neuromuscular symptoms arise because hypercalcemia decreases neuromuscular excitability, stabilizing cell membranes and raising the threshold for depolarization. Patients experience generalized weakness and fatigue. As calcium rises further, lethargy and central nervous system depression become apparent. Confusion develops in severe hypercalcemia, and coma can occur with very severe elevations, representing a life-threatening emergency requiring immediate intervention.

Cardiovascular manifestations of hypercalcemia are clinically important and detectable on electrocardiography. The characteristic ECG finding is a shortened QT interval, reflecting accelerated cardiac repolarization. Bradycardia may develop, and hypertension is commonly associated with chronic hypercalcemia. Of particular clinical significance, hypercalcemia increases sensitivity to digitalis, raising the risk of digitalis toxicity even at therapeutic drug levels, which demands careful monitoring in patients taking these medications.

Renal effects of hypercalcemia include polyuria resulting from nephrogenic diabetes insipidus, as elevated calcium impairs the kidney's concentrating ability by interfering with aquaporin-2 function in the collecting duct. Polydipsia follows as a compensatory response to the urinary water losses. Nephrolithiasis develops from the chronic excretion of excess calcium in the urine, forming calcium-containing kidney stones. Progressive renal insufficiency can result from nephrocalcinosis and the chronic effects of hypercalcemia on the renal parenchyma. Gastrointestinal symptoms are equally prominent: constipation occurs due to decreased smooth muscle tone, nausea and vomiting are common, and anorexia frequently develops. Pancreatitis can occur in severe hypercalcemia, and peptic ulcer disease is particularly associated with MEN1 syndrome, where hyperparathyroidism coexists with gastrin-secreting pancreatic tumors.

<image>Panel A: Stones, Bones, Groans, Moans mnemonic showing stones (nephrolithiasis, nephrocalcinosis), bones (bone pain, fractures, osteitis fibrosa cystica), groans (abdominal pain, constipation, pancreatitis, PUD), and moans (psychiatric symptoms including depression and confusion). Panel B: Neuromuscular and CNS effects showing decreased neuromuscular excitability causing weakness, fatigue, lethargy, confusion, and coma in severe hypercalcemia. Panel C: Cardiovascular findings showing short QT interval on ECG, potential bradycardia, hypertension, and increased sensitivity to digitalis toxicity. Panel D: Renal and GI effects showing polyuria from nephrogenic diabetes insipidus (impaired concentrating ability), polydipsia, nephrolithiasis, renal insufficiency, constipation, nausea and vomiting, and pancreatitis in severe cases.</image>

III. Primary Hyperparathyroidism

Primary hyperparathyroidism is the most common cause of hypercalcemia in the outpatient setting and represents autonomous overproduction of PTH by one or more parathyroid glands. The incidence is approximately 25-30 per 100,000, with a strong female predominance of 3:1 over males. Peak incidence occurs between 50 and 60 years of age. In modern practice, the majority of patients present asymptomatically, with hypercalcemia discovered as an incidental finding on routine blood work rather than through the classic "stones, bones, groans, and moans" presentation that was historically more common before automated chemistry panels became standard.

The etiology of primary hyperparathyroidism is dominated by a single parathyroid adenoma, which accounts for 80-85% of cases. Four-gland hyperplasia represents the next most common cause at 10-15% of cases and is particularly associated with hereditary syndromes such as MEN1. Multiple adenomas account for 2-5% of cases, and parathyroid carcinoma is rare, occurring in less than 1% of patients. The distinction between adenoma and hyperplasia is clinically significant because it determines the surgical approach, with single adenomas amenable to minimally invasive focused parathyroidectomy while four-gland hyperplasia typically requires bilateral neck exploration.

The laboratory findings in primary hyperparathyroidism follow a characteristic pattern that enables diagnosis. Serum calcium is elevated, and the PTH level is either frankly elevated or "inappropriately normal" in the setting of hypercalcemia, since normal physiology would suppress PTH when calcium is high. Serum phosphorus is low or normal, reflecting the phosphaturic effect of PTH on the proximal tubule. Chloride is elevated due to PTH-induced bicarbonate wasting in the proximal tubule, creating a mild hyperchloremic metabolic acidosis. The 25(OH)D level is often low, as PTH stimulates conversion to the active 1,25(OH)2D form, depleting the 25(OH)D substrate. Urine calcium is usually elevated, reflecting the filtered load exceeding tubular reabsorption capacity despite PTH-enhanced reabsorption.

Complications of untreated or longstanding primary hyperparathyroidism affect multiple organ systems. Nephrolithiasis with calcium-containing stones is one of the most common presenting complaints. Osteoporosis develops, preferentially affecting cortical bone such as the distal radius (forearm), which distinguishes it from postmenopausal osteoporosis that preferentially affects trabecular bone. Osteitis fibrosa cystica, the classic skeletal manifestation with bone cysts, brown tumors, and subperiosteal resorption, was once a hallmark of the disease but is now rare due to early detection. Progressive renal impairment can develop from nephrocalcinosis. Cardiovascular complications include hypertension and left ventricular hypertrophy. Several genetic syndromes are associated with primary hyperparathyroidism: MEN1 (MEN1 gene) causes hyperparathyroidism with pituitary tumors and pancreatic neuroendocrine tumors; MEN2A (RET gene) includes hyperparathyroidism with medullary thyroid carcinoma and pheochromocytoma; HPT-JT syndrome (CDC73 gene) features hyperparathyroidism with jaw tumors (ossifying fibromas); and familial hypocalciuric hypercalcemia (FHH, CASR gene) is a benign condition for which surgery is not indicated.

<image>Panel A: Etiology pie chart showing single adenoma (80-85%), four-gland hyperplasia (10-15%), multiple adenomas (2-5%), and parathyroid carcinoma (less than 1%) with epidemiology data (women 3:1, peak age 50-60). Panel B: Laboratory findings pattern showing elevated calcium, elevated or inappropriately normal PTH, low or normal phosphorus, elevated chloride, low 25(OH)D, and usually elevated urine calcium. Panel C: Complications showing nephrolithiasis (calcium stones), osteoporosis preferentially affecting cortical bone (forearm), osteitis fibrosa cystica (classic but now rare), renal impairment, and cardiovascular effects (hypertension, LVH). Panel D: Genetic syndromes table showing MEN1 (MEN1 gene, HPT plus pituitary plus pancreatic NETs), MEN2A (RET gene, HPT plus MTC plus pheochromocytoma), HPT-JT (CDC73 gene, HPT plus jaw tumors), and FHH (CASR gene, benign condition not requiring surgery).</image>

IV. Primary Hyperparathyroidism - Diagnosis and Management

The diagnostic workup for primary hyperparathyroidism begins with biochemical confirmation and proceeds through localization studies before surgical intervention. Serum calcium confirms the presence of hypercalcemia. The PTH level should be elevated or inappropriately normal in the setting of elevated calcium, which is the hallmark finding distinguishing primary hyperparathyroidism from other causes of hypercalcemia. A 24-hour urine calcium collection is essential to distinguish primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH). Serum creatinine assesses renal function. The 25(OH)D level evaluates vitamin D status, which is often depleted. DEXA scanning measures bone mineral density and should include the forearm site, which is preferentially affected by PTH-mediated cortical bone loss.

The distinction between primary hyperparathyroidism and familial hypocalciuric hypercalcemia is critically important because FHH is a benign condition caused by an autosomal dominant inactivating mutation in the calcium-sensing receptor (CASR) that does not require surgical intervention. In primary hyperparathyroidism, PTH is elevated, urine calcium excretion is increased, and the calcium-to-creatinine clearance ratio is greater than 0.02. In FHH, PTH is normal or only slightly elevated, urine calcium excretion is characteristically low, and the calcium-to-creatinine clearance ratio is less than 0.01. Misdiagnosing FHH as primary hyperparathyroidism leads to unnecessary and ineffective surgery, since removing parathyroid tissue does not correct the fundamental receptor defect.

Localization studies are performed prior to surgery to guide the operative approach, but they are not used for diagnosis. The sestamibi scan is the most commonly used modality, exploiting the differential uptake and washout of the radiotracer in hyperfunctioning parathyroid tissue compared to thyroid tissue. Neck ultrasound provides complementary anatomic information and is operator-dependent. Four-dimensional CT (4D-CT) has emerged as a modality with higher sensitivity for adenoma detection, though it involves radiation exposure. MRI serves as an alternative when other modalities are inconclusive or contraindicated. These studies are meant to guide minimally invasive surgical approaches and should not be used in lieu of biochemical confirmation for making the diagnosis.

Current guidelines establish clear indications for parathyroidectomy in primary hyperparathyroidism. Surgery is always indicated in symptomatic patients, including those with nephrolithiasis or fractures. In asymptomatic patients, surgical criteria include serum calcium greater than 1 mg/dL above the upper limit of normal, age less than 50 years, creatinine clearance less than 60 mL/min, bone density T-score less than -2.5 at any site, 24-hour urine calcium greater than 400 mg/day with increased stone risk, and situations where long-term monitoring is not feasible or per patient preference. Surgical options include minimally invasive parathyroidectomy for patients with a localized single adenoma and bilateral neck exploration for four-gland hyperplasia or when preoperative localization has failed. Intraoperative PTH monitoring is used to confirm successful removal, with a 50% drop from the pre-excision baseline indicating adequate resection. The cure rate with experienced surgeons is 95-98%.

For patients who are not surgical candidates or who decline surgery, medical management focuses on mitigating the effects of hypercalcemia. Adequate hydration should be maintained to prevent nephrolithiasis and support renal function. Thiazide diuretics must be avoided because they increase calcium reabsorption in the distal tubule and worsen hypercalcemia. Paradoxically, adequate vitamin D supplementation is recommended, as deficiency stimulates further PTH secretion and worsens the hyperparathyroid state. Bisphosphonates are used for osteoporosis to reduce fracture risk. Cinacalcet, a calcimimetic agent that activates the calcium-sensing receptor on parathyroid cells, effectively lowers both PTH and serum calcium and is the primary pharmacologic agent for managing hypercalcemia in non-surgical candidates.

<image>Panel A: Diagnostic workup showing serum calcium confirmation, PTH measurement, 24-hour urine calcium to distinguish from FHH (calcium-to-creatinine clearance ratio greater than 0.02 for HPT versus less than 0.01 for FHH), and DEXA scan including forearm site. Panel B: Surgical indications showing symptomatic disease (always), calcium greater than 1 mg/dL above normal, age less than 50, creatinine clearance less than 60, T-score less than negative 2.5, and urine calcium greater than 400 mg/day with stone risk. Panel C: Localization studies and surgical options showing sestamibi scan, neck ultrasound, 4D-CT for adenoma localization (to guide minimally invasive parathyroidectomy), bilateral neck exploration for hyperplasia, and intraoperative PTH monitoring (50% drop confirms success) with 95-98% cure rate. Panel D: Medical management for non-surgical candidates showing adequate hydration, avoiding thiazides, adequate vitamin D supplementation, bisphosphonates for osteoporosis, and cinacalcet (calcimimetic) to decrease PTH and calcium.</image>

V. Secondary and Tertiary Hyperparathyroidism

Secondary hyperparathyroidism is defined as an appropriate, compensatory elevation in PTH secretion in response to hypocalcemia or other stimuli that lower ionized calcium. Unlike primary hyperparathyroidism where PTH secretion is autonomous, in secondary hyperparathyroidism the parathyroid glands are responding normally to a physiologic signal. Serum calcium is characteristically low or normal, and PTH is elevated in a compensatory fashion. The most common cause is chronic kidney disease (CKD), followed by vitamin D deficiency. The distinction from primary hyperparathyroidism is straightforward: in secondary disease, the parathyroid glands are reacting to an identifiable stimulus, and treating the underlying cause reduces PTH levels.

The pathophysiology of CKD-mineral bone disorder illustrates how progressive renal failure drives secondary hyperparathyroidism through multiple interconnected mechanisms. In early CKD, declining renal mass reduces 1-alpha-hydroxylase activity, decreasing production of active vitamin D (1,25(OH)2D), and PTH begins to rise. FGF23 levels increase early as an initial compensatory response attempting to maintain phosphate homeostasis by promoting renal phosphate excretion. As CKD progresses, phosphate excretion becomes increasingly impaired and serum phosphorus rises, while calcium falls due to decreased intestinal absorption from reduced active vitamin D and direct phosphate-calcium complexing. In end-stage renal disease (ESRD), severe secondary hyperparathyroidism develops with markedly elevated PTH driving renal osteodystrophy, a spectrum of bone disease that includes osteitis fibrosa cystica from high bone turnover, adynamic bone disease from over-suppression of PTH, and mixed uremic osteodystrophy.

Treatment of secondary hyperparathyroidism in CKD targets each element of the pathophysiologic cascade. Phosphate binders taken with meals reduce serum phosphorus by binding dietary phosphate in the gut and preventing its absorption. Active vitamin D in the form of calcitriol or vitamin D analogs such as paricalcitol and doxercalciferol suppress PTH secretion and increase intestinal calcium absorption. Cinacalcet activates the calcium-sensing receptor on parathyroid cells, lowering PTH secretion directly via the CaSR without raising calcium or phosphorus. Dietary phosphate restriction is an important adjunct to limit phosphorus intake at its source.

Tertiary hyperparathyroidism represents the transition from compensatory to autonomous PTH secretion after prolonged secondary hyperparathyroidism. When parathyroid glands are subjected to chronic stimulation over months to years, they undergo hyperplasia and eventually develop autonomous function, secreting PTH independently of calcium levels. The hallmark is hypercalcemia that is no longer suppressible by correcting the original stimulus. This condition is most commonly encountered after renal transplantation, when restoration of normal kidney function corrects the hypocalcemia and hyperphosphatemia that were driving secondary hyperparathyroidism, yet the hyperplastic, autonomous parathyroid glands continue to secrete excessive PTH. Treatment often requires parathyroidectomy because the glands have become refractory to medical suppression, though cinacalcet may be tried initially.

<image>Panel A: Secondary hyperparathyroidism definition showing appropriate compensatory PTH elevation in response to hypocalcemia, with low or normal calcium and elevated PTH, most commonly caused by chronic kidney disease and vitamin D deficiency. Panel B: CKD-mineral bone disorder pathophysiology showing decreased 1-alpha-hydroxylase (decreased active vitamin D), decreased phosphate excretion (increased serum phosphorus), increased FGF23, hypocalcemia from decreased absorption, and compensatory PTH elevation progressing to renal osteodystrophy. Panel C: Treatment ladder for secondary HPT in CKD showing phosphate binders, active vitamin D (calcitriol) or analogs (paricalcitol, doxercalciferol), cinacalcet for PTH suppression via CaSR, and dietary phosphate restriction. Panel D: Tertiary hyperparathyroidism showing autonomous PTH secretion after prolonged secondary HPT, hypercalcemia that is not suppressible, typical presentation after renal transplant when stimulus is removed but parathyroids remain hyperplastic and autonomous, often requiring parathyroidectomy.</image>

VI. Hypercalcemia of Malignancy

Hypercalcemia of malignancy is the most common cause of elevated calcium in hospitalized patients and occurs through several distinct mechanisms. The dominant mechanism, accounting for approximately 80% of cases, is humoral hypercalcemia of malignancy mediated by PTHrP (parathyroid hormone-related peptide). Local osteolytic metastases cause approximately 20% of cases through direct bone destruction. Production of 1,25(OH)2D by tumor tissue accounts for less than 1% of cases, occurring primarily in lymphomas and certain solid tumors. Ectopic PTH secretion by tumors is exceedingly rare and limited to individual case reports. Understanding these mechanisms is essential because each produces a distinct laboratory pattern and guides treatment.

PTHrP-mediated humoral hypercalcemia of malignancy is by far the most common mechanism. PTHrP is a protein that binds to the PTH receptor and mimics many of the actions of PTH, including increasing bone resorption and renal calcium reabsorption while promoting phosphate excretion. The laboratory pattern is distinctive: calcium is elevated, PTH is suppressed (because the native parathyroid glands are appropriately responding to hypercalcemia), phosphorus is low (reflecting the phosphaturic effect of PTHrP on the PTH receptor), and PTHrP levels are measurably elevated. The tumors most commonly associated with PTHrP production include squamous cell carcinomas of the lung and head and neck, breast cancer, renal cell carcinoma, and bladder cancer. The prognosis is generally poor because PTHrP-mediated hypercalcemia usually indicates advanced, widespread disease.

Local osteolytic hypercalcemia accounts for the remaining 20% of malignancy-associated cases and results from tumor cells metastatic to bone releasing cytokines that activate osteoclasts and destroy bone locally. The key mediators include RANKL, interleukin-6, tumor necrosis factor, and other osteoclast-activating factors. The laboratory pattern shows elevated calcium with suppressed PTH, but PTHrP is characteristically normal, distinguishing this mechanism from humoral hypercalcemia. The tumors most commonly associated with this mechanism include breast cancer with bone metastases, multiple myeloma, and lung cancer with skeletal metastases.

Treatment of hypercalcemia of malignancy follows a stepwise approach guided by severity. Intravenous normal saline is the first-line intervention, restoring intravascular volume and promoting calciuresis through saline diuresis. Bisphosphonates are the mainstay of treatment, with zoledronic acid preferred due to its superior efficacy and convenience; pamidronate is an alternative. Denosumab, a RANKL inhibitor, is used for bisphosphonate-refractory cases. Calcitonin provides rapid onset calcium lowering within hours but its effect is transient, lasting only 48-72 hours due to tachyphylaxis. Treatment of the underlying malignancy is essential for sustained calcium control. Glucocorticoids are particularly effective for hypercalcemia associated with lymphoma, myeloma, and vitamin D-mediated mechanisms, as they inhibit the 1-alpha-hydroxylase activity in tumor macrophages and reduce tumor cytokine production.

<image>Panel A: PTHrP-mediated humoral hypercalcemia of malignancy (80% of cases) showing PTHrP binding PTH receptor to mimic PTH actions, with laboratory pattern of elevated calcium, suppressed PTH, low phosphorus, and elevated PTHrP in squamous cell, breast, and renal cancers. Panel B: Local osteolytic hypercalcemia (20%) showing tumor cells in bone releasing cytokines (RANKL, IL-6, TNF) to activate osteoclasts, with elevated calcium, suppressed PTH, and normal PTHrP in breast cancer, myeloma, and lung metastases. Panel C: Rare mechanisms including 1,25(OH)2D production by lymphomas (less than 1%) and very rare ectopic PTH secretion. Panel D: Treatment approach showing IV fluid resuscitation as first-line, bisphosphonates (zoledronic acid preferred), denosumab for bisphosphonate-refractory cases, calcitonin for rapid but transient effect, treatment of underlying malignancy, and glucocorticoids for lymphoma, myeloma, and vitamin D-mediated hypercalcemia.</image>

VII. Hypocalcemia - Overview

Hypocalcemia is classified by severity based on total serum calcium levels, with each tier carrying different clinical implications and treatment urgency. Mild hypocalcemia is defined as a total calcium of 7.5-8.5 mg/dL, moderate hypocalcemia as 7-7.5 mg/dL, and severe hypocalcemia as less than 7 mg/dL. Severe hypocalcemia constitutes a medical emergency due to the risk of life-threatening complications including seizures, laryngospasm, and cardiac arrhythmias, requiring immediate intravenous calcium replacement and cardiac monitoring.

The causes of hypocalcemia can be organized by the underlying pathophysiologic mechanism. Conditions with low PTH include surgical hypoparathyroidism (the most common cause), autoimmune destruction of the parathyroid glands, and infiltrative diseases. PTH resistance, as seen in pseudohypoparathyroidism, results in hypocalcemia despite elevated PTH because target organs fail to respond normally. Vitamin D deficiency encompasses nutritional deficiency, malabsorption syndromes, and impaired activation in liver or kidney disease. Other causes include acute pancreatitis (where calcium is sequestered through saponification with fatty acids), hungry bone syndrome (rapid calcium uptake by bones following parathyroidectomy), and various medications.

The PTH level provides the most important branch point in the differential diagnosis of hypocalcemia. When PTH is low in the setting of hypocalcemia, the diagnosis is hypoparathyroidism, since the parathyroid glands are failing to mount the appropriate compensatory response. When PTH is appropriately elevated in response to hypocalcemia, the clinician should consider vitamin D deficiency, PTH resistance (pseudohypoparathyroidism), or chronic kidney disease as the underlying cause, since in each of these conditions the parathyroid glands are functioning normally but the hypocalcemia persists due to a downstream problem. This PTH-based framework efficiently narrows the differential and guides further workup.

<image>Panel A: Hypocalcemia severity classification showing mild (7.5-8.5 mg/dL), moderate (7-7.5 mg/dL), and severe (less than 7 mg/dL) total calcium levels. Panel B: Causes organized by category showing low PTH (surgical, autoimmune, infiltrative hypoparathyroidism), PTH resistance (pseudohypoparathyroidism), vitamin D deficiency (nutritional, malabsorption, liver or kidney disease), and other (pancreatitis, hungry bone syndrome, medications). Panel C: PTH-based diagnostic approach showing low PTH directing to hypoparathyroidism diagnosis versus high PTH directing to vitamin D deficiency, PTH resistance, or CKD as the cause. Panel D: Key distinguishing features in each category with expected laboratory patterns for PTH, calcium, phosphorus, and vitamin D levels to guide differential diagnosis.</image>

VIII. Hypocalcemia - Clinical Features

The clinical manifestations of hypocalcemia are dominated by neuromuscular hyperexcitability, which reflects the fundamental role of extracellular calcium in stabilizing voltage-gated sodium channels and setting the threshold for nerve and muscle depolarization. When calcium falls, the threshold for depolarization decreases, making neurons and muscle fibers fire more easily. Tetany, characterized by involuntary muscle spasms, is the hallmark of symptomatic hypocalcemia. The Chvostek sign is elicited by tapping over the facial nerve anterior to the ear, producing ipsilateral twitching of the facial muscles, though this sign can be present in up to 10% of normocalcemic individuals. The Trousseau sign, which is more specific, is elicited by inflating a blood pressure cuff above systolic pressure for three minutes, producing carpal spasm with flexion of the wrist, metacarpophalangeal joints, and extension of the interphalangeal joints. Carpopedal spasm involves involuntary contraction of the muscles of the hands and feet. Paresthesias, particularly perioral tingling and fingertip numbness, are often the earliest symptoms. Seizures can occur with severe hypocalcemia and may be the presenting feature. Laryngospasm, causing stridor and airway compromise, represents a true emergency.

Cardiovascular manifestations of hypocalcemia are clinically significant and require monitoring. The prolonged QT interval is the characteristic ECG finding, reflecting delayed cardiac repolarization and carrying an associated risk of torsades de pointes and other arrhythmias. In severe, chronic hypocalcemia, heart failure can develop due to impaired myocardial contractility. Acute hypotension may occur as calcium is required for normal vascular smooth muscle tone and cardiac function.

Psychiatric and central nervous system symptoms accompany hypocalcemia with varying severity. Anxiety is a common early complaint and may reflect the heightened neuronal excitability. Chronic hypocalcemia can produce persistent depression. Confusion develops with more severe calcium depletion and indicates significant CNS dysfunction. Psychosis, while rare, has been reported in severe or longstanding hypocalcemia and may be mistaken for a primary psychiatric disorder if the underlying metabolic abnormality is not identified.

Chronic hypocalcemia produces additional manifestations that develop over months to years. Dental abnormalities including enamel hypoplasia and defective root formation occur when hypocalcemia is present during childhood development. Cataracts develop in chronic hypocalcemia due to calcium deposition in the lens. Basal ganglia calcifications are a characteristic finding in long-standing hypoparathyroidism, visible on CT imaging, resulting from dystrophic calcium-phosphate deposition in this region. Ectodermal changes including dry skin and brittle nails reflect the role of calcium in epithelial cell function and differentiation.

<image>Panel A: Neuromuscular excitability features showing tetany (muscle spasms), Chvostek sign (facial twitch with tapping over facial nerve), Trousseau sign (carpal spasm with blood pressure cuff inflation), carpopedal spasm, perioral and fingertip paresthesias, seizures, and laryngospasm. Panel B: Cardiovascular findings showing prolonged QT interval on ECG with risk of arrhythmia, heart failure in severe chronic cases, and acute hypotension. Panel C: Psychiatric and CNS symptoms showing anxiety, depression, confusion, and rare psychosis with increasing severity of hypocalcemia. Panel D: Chronic hypocalcemia features showing dental abnormalities (if childhood onset), cataracts, basal ganglia calcifications (long-standing hypoparathyroidism), and ectodermal changes including dry skin and brittle nails.</image>

IX. Hypoparathyroidism

Hypoparathyroidism, defined as insufficient PTH secretion leading to hypocalcemia and hyperphosphatemia, has a range of etiologies that are dominated by iatrogenic causes. Post-surgical hypoparathyroidism accounts for approximately 75% of all cases and results from inadvertent damage to or removal of the parathyroid glands during thyroid or parathyroid surgery. Autoimmune destruction of parathyroid tissue is the second most common cause at approximately 15%. Genetic causes account for approximately 5% of cases. The remaining 5% includes infiltrative diseases such as hemochromatosis, Wilson disease, or metastatic cancer, radiation damage, and magnesium deficiency, which functionally impairs PTH secretion and action.

Post-surgical hypoparathyroidism occurs most commonly after total thyroidectomy or bilateral parathyroid surgery, procedures that place the parathyroid glands at risk due to their small size and variable anatomic location in close proximity to the thyroid. Transient hypoparathyroidism is common following these procedures and typically recovers over weeks to months as residual parathyroid tissue regains normal function, often from recovery of devascularized but not completely destroyed glands. Permanent hypoparathyroidism is defined as persistent disease lasting more than 6 months and occurs when insufficient parathyroid tissue remains viable. Prevention strategies during surgery include meticulous identification and preservation of the parathyroid glands and their blood supply, and autotransplantation of parathyroid tissue into the sternocleidomastoid muscle or forearm if a gland is inadvertently devascularized.

Autoimmune hypoparathyroidism may occur as an isolated condition from autoimmune destruction of the parathyroid glands, or as part of autoimmune polyglandular syndrome type 1 (APS type 1), a rare autosomal recessive condition caused by mutations in the AIRE (autoimmune regulator) gene. APS type 1 is characterized by the classic triad of hypoparathyroidism, Addison disease (adrenal insufficiency), and chronic mucocutaneous candidiasis. Hypoparathyroidism is typically the first endocrine manifestation to appear, often in childhood. DiGeorge syndrome is the most important genetic cause of hypoparathyroidism, resulting from a microdeletion at chromosome 22q11.2 that leads to absent or hypoplastic parathyroid glands along with thymic hypoplasia. The features are captured by the CATCH-22 mnemonic: Cardiac defects (especially conotruncal abnormalities), Abnormal facies, Thymic hypoplasia (leading to T-cell immunodeficiency), Cleft palate, and Hypocalcemia, with 22 referencing the chromosome involved.

The relationship between magnesium and PTH is critically important in the workup and management of hypocalcemia. Low magnesium impairs PTH secretion from the parathyroid glands, as magnesium is required for normal exocytosis of PTH-containing secretory granules. Very low magnesium additionally causes PTH resistance at target organs, meaning that even if some PTH is secreted, the kidneys and bones fail to respond normally. The clinical implication is paramount: hypocalcemia in the setting of hypomagnesemia will not respond to calcium replacement alone, and magnesium must be corrected first before PTH secretion and action can normalize and calcium homeostasis can be restored.

<image>Panel A: Causes of hypoparathyroidism showing post-surgical (75%, from thyroid or parathyroid surgery with transient or permanent forms), autoimmune (15%), genetic (5%), and other causes (5%, infiltrative, radiation, magnesium deficiency). Panel B: DiGeorge syndrome (22q11.2 deletion) showing CATCH-22 mnemonic with cardiac defects, abnormal facies, thymic hypoplasia, cleft palate, and hypocalcemia from absent or hypoplastic parathyroid glands. Panel C: Autoimmune polyglandular syndrome type 1 (APS-1, AIRE mutation) showing triad of hypoparathyroidism, Addison disease, and mucocutaneous candidiasis with autoimmune destruction of parathyroid tissue. Panel D: Magnesium and PTH relationship showing low magnesium impairing PTH secretion, very low magnesium causing PTH resistance at target organs, and the clinical importance of correcting magnesium first before hypocalcemia will respond to treatment.</image>

X. Treatment of Hypocalcemia

Acute, severe hypocalcemia requires emergent intravenous calcium replacement when patients present with symptomatic tetany, seizures, cardiac arrhythmias, or a total calcium less than 7.5 mg/dL. The treatment of choice is intravenous calcium gluconate, administered as a 10% solution with a bolus of 10-20 mL given over 10-20 minutes, followed by a continuous infusion at a rate of 1-2 mg/kg/hr of elemental calcium. Serum calcium must be monitored every 4-6 hours to guide the infusion rate and avoid overcorrection, and continuous cardiac monitoring is essential given the arrhythmogenic potential of both severe hypocalcemia and rapid calcium correction. Calcium gluconate is preferred over calcium chloride for peripheral intravenous administration because it is safer with respect to tissue damage; calcium chloride can cause severe tissue necrosis if extravasation occurs, though it delivers more elemental calcium per volume and may be preferred through central venous access in emergencies. Regardless of formulation, extreme care must be taken to avoid extravasation, which can cause significant local tissue injury.

Chronic hypocalcemia management in hypoparathyroidism centers on oral calcium supplementation combined with active vitamin D. Calcium supplementation at doses of 1-3 g per day of elemental calcium, typically divided into multiple doses to optimize absorption, provides the substrate for maintaining serum calcium levels. Calcitriol (1,25-dihydroxyvitamin D) is the preferred form of vitamin D in hypoparathyroidism because these patients lack PTH and therefore cannot adequately activate vitamin D through the PTH-dependent 1-alpha-hydroxylase enzyme in the kidney. Calcitriol, being the already-active form, bypasses this requirement entirely. Typical doses range from 0.25 to 2 micrograms per day, titrated to maintain serum calcium in the low-normal range. The treatment target is deliberately set at low-normal calcium rather than mid-normal because these patients lack PTH-mediated renal calcium reabsorption; targeting higher levels increases the risk of hypercalciuria and nephrolithiasis. Urine calcium should be monitored regularly with a target of less than 250 mg per 24 hours.

Recombinant PTH (1-84), marketed as Natpara, represents a significant advance in the treatment of refractory hypoparathyroidism. This hormone replacement therapy is indicated for patients in whom high-dose calcium and vitamin D supplementation are inadequate to maintain acceptable calcium levels or produce unacceptable hypercalciuria. The benefit of PTH replacement is a reduction in the required doses of calcium and vitamin D supplements, with the physiologic advantage of restoring PTH-mediated renal calcium reabsorption and reducing urinary calcium losses. Monitoring includes regular assessment of serum calcium and urine calcium levels.

Pseudohypoparathyroidism must be distinguished from true hypoparathyroidism because its pathophysiology and laboratory findings are fundamentally different, even though both produce hypocalcemia. Pseudohypoparathyroidism is defined by end-organ resistance to PTH, meaning that the parathyroid glands produce PTH normally but the kidneys and bones fail to respond. The laboratory pattern is characteristic: calcium is low, phosphorus is high (because renal phosphate excretion is not stimulated), and PTH is markedly elevated as the glands attempt to compensate for the resistance. Type 1a pseudohypoparathyroidism is associated with Albright hereditary osteodystrophy (AHO), a distinctive phenotype caused by inactivating mutations in the GNAS gene encoding the stimulatory G-protein alpha subunit (Gs-alpha) that couples the PTH receptor to adenylyl cyclase. AHO features include short stature, round face, brachydactyly (particularly shortened fourth and fifth metacarpals), subcutaneous ossifications, and intellectual disability. Treatment consists of calcium supplementation combined with calcitriol, following the same principles as in hypoparathyroidism, to bypass the defective PTH signaling pathway and maintain adequate calcium levels through direct supplementation.

<image>Panel A: Acute hypocalcemia treatment protocol showing IV calcium gluconate 10% bolus (10-20 mL over 10-20 minutes) followed by continuous infusion (1-2 mg/kg/hr elemental calcium) with cardiac monitoring and serum calcium checks every 4-6 hours, noting gluconate is safer than chloride for tissue. Panel B: Chronic hypocalcemia management showing calcium supplementation (1-3 g/day elemental), calcitriol as preferred vitamin D form in hypoparathyroidism (0.25-2 mcg/day, bypasses need for PTH-dependent activation), targeting low-normal calcium to avoid hypercalciuria (less than 250 mg/24h). Panel C: PTH replacement therapy showing recombinant PTH 1-84 (Natpara) for refractory hypoparathyroidism when high-dose calcium and vitamin D are inadequate, reducing calcium and vitamin D requirements with calcium and urine calcium monitoring. Panel D: Pseudohypoparathyroidism showing PTH resistance with low calcium, high phosphorus, and high PTH, type 1a (Albright hereditary osteodystrophy with short stature, round face, brachydactyly, GNAS mutation), treated with calcium and calcitriol supplementation.</image>

Summary

• Hypercalcemia: Primary HPT most common outpatient; malignancy most common inpatient
• Primary HPT: Elevated PTH with hypercalcemia; adenoma (85%); surgery if indicated
• Secondary HPT: Appropriate PTH elevation in response to hypocalcemia (CKD, vitamin D deficiency)
• Hypercalcemia of malignancy: PTHrP (humoral) or local osteolysis; treat with fluids + bisphosphonates
• Hypocalcemia: Low PTH = hypoparathyroidism; high PTH = vitamin D deficiency or resistance
• Hypoparathyroidism: Post-surgical most common; treat with calcium + calcitriol
• Clinical signs: Chvostek, Trousseau (hypocalcemia); short QT (hypercalcemia)

Key Terms

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