Parathyroid gland (more focus on hyperparathyroidism)

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Parathyroid Glands and Hyperparathyroidism

Source: Robbins, Cotran & Kumar - Pathologic Basis of Disease, pp. 1010-1013

1. Normal Parathyroid Gland Anatomy & Histology

The parathyroid glands develop from the pharyngeal pouches (same pouches that give rise to the thymus). There are four glands, located near the upper and lower poles of each thyroid lobe, but ectopic locations are possible - the carotid sheath, thymus, and anterior mediastinum.

Two cell types:

Cell Type	Characteristics	Function
Chief cells	Predominate; 12-20 µm, polygonal, round central nuclei, light-to-dark pink cytoplasm	Contain secretory granules with PTH
Oxyphil cells	Slightly larger, eosinophilic cytoplasm, packed with mitochondria, sparse secretory granules	Large number of calcium-sensing receptors; produce PTHrP and calcitriol

In early childhood, glands are almost entirely chief cells. Stromal fat increases until age ~25, reaching a maximum of ~30% of gland volume.

2. PTH - Functions (Calcium Homeostasis)

The parathyroids respond to free (ionized) calcium in the blood. Low calcium → PTH release. PTH acts via four mechanisms:

Renal tubular reabsorption of calcium (conserving free Ca²⁺)
Activates vitamin D (1α-hydroxylase in kidney) → increases intestinal calcium absorption
Increases urinary phosphate excretion → lowers serum phosphate → further raises ionized Ca²⁺
Stimulates osteoclast differentiation → bone resorption → releases ionized calcium

The net effect is elevated free calcium, which then inhibits further PTH secretion via classic negative feedback.

3. Hyperparathyroidism - Overview

Hyperparathyroidism is classified into three types:

Type	Mechanism
Primary	Autonomous overproduction of PTH (adenoma/hyperplasia)
Secondary	Compensatory hypersecretion due to chronic hypocalcemia (renal failure, Vit D deficiency)
Tertiary	Autonomous PTH production developing after long-standing secondary HPT

4. Primary Hyperparathyroidism (1° HPT)

Epidemiology

One of the most common endocrine disorders
Annual incidence: ~25 cases per 100,000 in US/Europe
Female:male ratio = nearly 4:1
Most patients are in their 50s or older
Up to 80% are asymptomatic - discovered incidentally on serum electrolyte panels

Causes (by frequency)

Adenoma: 85-95% (solitary, almost always sporadic)
Primary hyperplasia (diffuse or nodular): 5-10%
Parathyroid carcinoma: ~1%

Molecular Pathogenesis of Sporadic Adenomas

1. Cyclin D1 (CCND1) gene inversions: A pericentric inversion on chromosome 11 places CCND1 (normally on 11q) adjacent to the PTH gene (on 11p). The PTH gene's regulatory elements then drive overexpression of cyclin D1, a major cell cycle regulator - causing uncontrolled proliferation. Cyclin D1 is overexpressed in ~40% of parathyroid adenomas even without CCND1 rearrangement.

2. MEN1 mutations: ~15% of sporadic tumors have somatic mutations in the MEN1 tumor suppressor gene (chromosome 11q13) with loss of heterozygosity (LOH). Germline MEN1 mutations cause MEN-1 syndrome.

3. CDC73 mutations: Mutated in ~70% of sporadic parathyroid carcinomas; encodes parafibromin (transcriptional regulator). Germline CDC73 mutations cause the HPT-JT syndrome (hyperparathyroidism-jaw tumor syndrome).

Familial Syndromes

Syndrome	Key Features
MEN-1	Parathyroid hyperplasia, pituitary tumors, pancreatic neuroendocrine tumors
MEN-2A	Parathyroid hyperplasia, medullary thyroid carcinoma, pheochromocytoma
MEN-4	Similar to MEN-1
FHH (Familial Hypocalciuric Hypercalcemia)	Heterozygous loss-of-function CASR mutations; elevated Ca²⁺ with non-suppressed PTH but NO downstream consequences - does NOT need surgery

Morphology

Parathyroid Adenoma:

Almost always solitary
Average size 0.5-5 g; well-circumscribed, soft, tan-reddish-brown nodule
Microscopically: sheets of uniform, polygonal chief cells with small central nuclei; nests of larger oxyphil cells also present
Adipose tissue is inconspicuous (unlike normal parathyroid)
A rim of compressed non-neoplastic parathyroid tissue at the edge (important diagnostic clue)
Mitotic figures are rare; bizarre nuclei may be seen (endocrine atypia - NOT a criterion for malignancy)
The other glands are normal or shrunken due to feedback inhibition

Fig. 24.24 - Tc-99m-sestamibi radionuclide scan showing increased uptake at the left inferior parathyroid (arrow), corresponding to a parathyroid adenoma. Preoperative scintigraphy distinguishes adenomas (single gland uptake) from hyperplasia (multiple glands).

Primary Hyperplasia:

May be sporadic or part of MEN syndrome
All four glands involved (though asymmetrically - making it hard to distinguish from adenoma)
Combined weight rarely exceeds 1 g
Chief cell hyperplasia most common (diffuse or multinodular); less commonly "water-clear cell hyperplasia"

Parathyroid Carcinoma:

Defined by invasive growth and/or metastasis (cytology is unreliable)
Involves single gland; gray-white irregular masses, sometimes >10 g
Local recurrence in 1/3 of cases; distant metastasis in another 1/3
Atypical parathyroid neoplasms: new entity; fibrosis, cystic change, macronucleoli without definitive invasion - many show parafibromin loss

Skeletal Changes (Morphology)

Untreated symptomatic 1° HPT leads to three interrelated skeletal abnormalities:

Osteoporosis - decreased bone mass, preferential involvement of phalanges, vertebrae, proximal femur; cortical bone affected more than medullary bone
Dissecting osteitis - osteoclasts tunnel along trabeculae creating "railroad track" appearance
Brown tumors - collections of osteoclasts, reactive giant cells, and hemorrhagic debris that replace normal bone; yellow-brown color (not a true neoplasm); associated with osteitis fibrosa cystica
Osteitis fibrosa cystica - end-stage: cystic spaces filled with fibrous tissue and hemosiderin-laden macrophages

Renal Changes (Morphology)

Nephrolithiasis (kidney stones) - most common renal complication; calcium oxalate or calcium phosphate stones
Nephrocalcinosis - calcium deposition in the renal parenchyma; can impair renal function

Clinical Features - The Classic Mnemonic

"Painful bones, renal stones, abdominal groans, and psychic moans"

System	Manifestations
Skeletal	Bone pain, pathological fractures, osteitis fibrosa cystica, brown tumors
Renal	Kidney stones (Ca-oxalate/phosphate), nephrocalcinosis, polyuria, polydipsia
GI (abdominal groans)	Nausea, vomiting, constipation, peptic ulcer disease (Ca²⁺ stimulates gastrin), pancreatitis
Neuropsychiatric (psychic moans)	Depression, cognitive dysfunction, anxiety, fatigue, weakness
Cardiovascular	Hypertension, shortened QT interval
Asymptomatic	~80% - incidentally discovered hypercalcemia

Lab Findings

Elevated serum calcium (hypercalcemia)
Elevated PTH (inappropriately high or frankly elevated)
Hypophosphatemia (PTH causes phosphaturia)
Hypercalciuria (increased filtered load overwhelms reabsorption)
Elevated alkaline phosphatase (if bone disease present)
Elevated urine cAMP (PTH receptor-mediated)

Differential of Hypercalcemia

Primary HPT is the most common cause of asymptomatic hypercalcemia. Malignancy is the most common cause of symptomatic hypercalcemia. Together they account for nearly 90% of all cases of hypercalcemia.

5. Secondary Hyperparathyroidism (2° HPT)

Causes of the underlying hypocalcemia:

Chronic renal failure (most common)
Vitamin D deficiency
Inadequate dietary calcium
Steatorrhea (fat malabsorption of vitamin D)

Mechanism in Renal Failure:

Decreased phosphate excretion → hyperphosphatemia
Elevated phosphate directly depresses serum calcium → stimulates parathyroid activity
Loss of renal α1-hydroxylase → reduced active vitamin D → decreased intestinal calcium absorption
Vitamin D deficiency removes its suppressive effect on parathyroid growth and PTH secretion → compound hypersecretion

Morphology

All parathyroid glands are hypercellular (asymmetric enlargement possible)
Increased chief cells or water-clear cells (diffuse or multinodular)
Fat cells decreased
Metastatic calcification in lungs, heart, stomach, blood vessels
Distinction between hyperplasia and adenoma may be impossible histologically

Clinical Features

Dominated by the underlying chronic renal failure
Renal osteodystrophy - bony changes that regress with treatment
Calciphylaxis - vascular calcification causing ischemic skin/organ damage (serious complication)
Responds to dietary vitamin D supplementation and phosphate binders

6. Tertiary Hyperparathyroidism

Develops after long-standing secondary HPT
Parathyroid activity becomes autonomous and excessive
Results in hypercalcemia (as opposed to normal/low calcium in 2° HPT)
May require parathyroidectomy

7. Hypoparathyroidism (Brief)

Most common cause: inadvertent removal or injury during thyroid surgery
Less common: autoimmune destruction, DiGeorge syndrome (developmental failure of pharyngeal pouches)
Results in hypocalcemia → tetany, seizures, Chvostek sign, Trousseau sign
Dental abnormalities in developmental hypocalcemia: hypoplasia, defective enamel, carious teeth

Pseudohypoparathyroidism: End-organ resistance to PTH. PTH levels are normal or elevated, but target tissues do not respond. Caused by G-protein pathway defects. Also causes resistance to TSH and FSH/LH. Presents with hypocalcemia, hyperphosphatemia, and elevated PTH.

Key Concepts Summary (Robbins)

1° HPT is the most common cause of asymptomatic hypercalcemia
Most cases are sporadic parathyroid adenomas (solitary)
Adenomas are monoclonal; hyperplasia is multiglandular
Skeletal triad: bone resorption, osteitis fibrosa cystica, brown tumors
Renal: nephrolithiasis and nephrocalcinosis
Clinical mnemonic: "painful bones, renal stones, abdominal groans, psychic moans"
2° HPT: hyperphosphatemia and low vitamin D in renal failure → compensatory PTH hypersecretion
Tertiary HPT: autonomous PTH secretion arising from 2° HPT

- Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 1010-1013

Explain hypo also

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Hypoparathyroidism

Source: Robbins, Cotran & Kumar - Pathologic Basis of Disease, pp. 1012-1013

Definition

Hypoparathyroidism is a state of PTH deficiency (or end-organ resistance to PTH), resulting in hypocalcemia and hyperphosphatemia. It is far less common than hyperparathyroidism.

Causes / Etiology

1. Surgical (Most Common Acquired Cause)

Inadvertent damage or removal of parathyroid glands during:

Thyroidectomy (total or near-total) - most common scenario
Radical neck dissection for malignancy (glands mistaken for lymph nodes)
Over-resection of parathyroid tissue during treatment of 1° HPT

2. Autoimmune Hypoparathyroidism

Associated with chronic mucocutaneous candidiasis and primary adrenal insufficiency
This triad is called Autoimmune Polyglandular Syndrome type 1 (APS-1)
Caused by mutations in the AIRE gene (autoimmune regulator)
Typical presentation sequence in childhood:
- Candidiasis (first, early childhood)
- Hypoparathyroidism (several years later)
- Adrenal insufficiency (adolescence)

3. Autosomal Dominant Hypoparathyroidism

Caused by gain-of-function mutations in the CASR gene (calcium-sensing receptor)
Heightened calcium sensing suppresses PTH inappropriately → hypocalcemia + hypercalciuria
Contrast with FHH (heterozygous loss-of-function CASR) which causes hypercalcemia

4. Familial Isolated Hypoparathyroidism (FIH)

Rare, autosomal dominant or recessive
AD form: mutation in the PTH gene itself - impairs processing to the active mature hormone
AR form: loss-of-function mutations in GCM2 (transcription factor essential for parathyroid development)

5. Congenital Absence of Parathyroid Glands

Associated with thymic aplasia and cardiovascular defects
Classic association: 22q11 deletion syndrome (DiGeorge syndrome) - failure of 3rd and 4th pharyngeal pouches to develop → absent parathyroids + absent thymus + cardiac defects

Pathophysiology

↓ PTH → ↓ renal calcium reabsorption + ↓ vitamin D activation + ↓ osteoclast activity → Hypocalcemia

↓ PTH → ↓ phosphate excretion → Hyperphosphatemia

High phosphate further binds ionized calcium and deposits in tissues.

Clinical Features

All manifestations stem from hypocalcemia - the severity depends on how low calcium falls and how quickly.

Neuromuscular (Most Prominent)

Tetany - the hallmark of hypocalcemia:

Circumoral numbness and paresthesias (tingling of lips, hands, feet) - earliest symptoms
Carpopedal spasm - classic flexion of wrist/fingers
Laryngospasm - life-threatening
Generalized seizures

Classic signs:

Sign	How to Elicit	What Happens
Chvostek sign	Tap facial nerve anterior to ear	Twitching of eye, mouth, or nose muscles (subclinical tetany)
Trousseau sign	Inflate BP cuff on arm above systolic for 3 minutes	Carpal spasm (wrist flexion, finger extension) - more specific

Neuropsychiatric

Emotional instability, anxiety, depression
Confusion, hallucinations
Frank psychosis

Intracranial

Basal ganglia calcification (paradoxically, hypocalcemia causes calcification due to elevated phosphate driving calcium phosphate deposition)
Parkinsonian-like movement disorders
Increased intracranial pressure → papilledema

Ocular

Lens calcification and cataract formation

Cardiovascular

Prolonged QT interval on ECG (as opposed to shortened QT in hypercalcemia)
Risk of ventricular arrhythmias

Dental (Developmental Hypocalcemia Only)

These changes are highly characteristic and occur when hypocalcemia is present during development:

Dental hypoplasia
Failure of eruption
Defective enamel and root formation
Abraded, carious teeth

Pseudohypoparathyroidism (PHP)

A separate entity - PTH levels are normal or elevated, but target organs fail to respond.

Mechanism: Genetic defects in the G-protein pathway (Gsα subunit - GNAS mutations) that is shared across multiple hormone receptors.

Result: End-organ resistance to PTH (and also TSH, FSH, LH - all G-protein-coupled receptors)

Feature	Hypoparathyroidism	Pseudohypoparathyroidism
PTH level	Low	Normal or High
Ca²⁺	Low	Low
Phosphate	High	High
Response to exogenous PTH	Normal (urine cAMP rises)	No response
TSH	Normal	May be elevated (TSH resistance)

Clinical: Hypocalcemia + hyperphosphatemia + elevated PTH. TSH resistance is mild. LH/FSH resistance → hypergonadotropic hypogonadism in females.

Quick Comparison Table: Hypo vs. Hyper

Feature	Hypoparathyroidism	Primary Hyperparathyroidism
PTH	↓	↑
Serum Ca²⁺	↓ (hypocalcemia)	↑ (hypercalcemia)
Serum PO₄	↑ (hyperphosphatemia)	↓ (hypophosphatemia)
Urine Ca²⁺	↓	↑
QT interval	Prolonged	Shortened
Key symptom	Tetany, seizures	Stones, bones, groans, moans
Bone disease	Osteomalacia (if chronic)	Osteitis fibrosa cystica, brown tumors

- Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 1012-1013

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