Pseudo hypo parathyroid

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pseudohypoparathyroidism pathophysiology types clinical features

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pseudohypoparathyroidism diagnosis management treatment Albright hereditary osteodystrophy

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pseudohypoparathyroidism type 1a 1b 2 GNAS mutation PTH resistance hypocalcemia

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pseudohypoparathyroidism Albright hereditary osteodystrophy features brachydactyly

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Here is a comprehensive overview of Pseudohypoparathyroidism (PHP):

Pseudohypoparathyroidism (PHP)

Definition

PHP is a group of inherited endocrine disorders characterized by end-organ resistance to parathyroid hormone (PTH) — unlike true hypoparathyroidism, where PTH is deficient. In PHP, PTH levels are elevated (or high-normal), yet the kidney and other target tissues fail to respond appropriately, resulting in hypocalcemia and hyperphosphatemia.

Pathophysiology

The defect lies in the PTH receptor signaling cascade:

PTH binds its receptor (PTH1R) → activates Gs-alpha (Gsα) protein → stimulates adenylyl cyclase → generates cAMP → downstream effects (phosphaturia, 1,25-OH₂D production, calcium reabsorption)
In PHP, a mutation or imprinting defect in the GNAS gene (encoding Gsα) impairs this cascade
Failure to produce 1,25(OH)₂D → impaired intestinal calcium absorption
Impaired phosphate excretion → hyperphosphatemia
Result: hypocalcemia despite elevated PTH

Classification

Type	Mechanism	AHO Features	PTH Resistance	Other Hormone Resistance
PHP Type Ia (PHP1A)	Heterozygous loss-of-function GNAS mutation (maternal allele)	Present	Yes	TSH, LH/FSH, GHRH resistance also
PHP Type Ib (PHP1B)	Imprinting defect at GNAS locus (methylation abnormality)	Absent	Yes	Primarily PTH; sometimes mild TSH
PHP Type Ic	Gsα activity normal in vitro but AHO phenotype present	Present	Yes	Similar to Ia
PHP Type II	Normal urinary cAMP response to PTH but no phosphaturia	Absent	Partial (post-cAMP defect)	Usually isolated
Pseudopseudohypoparathyroidism (PPHP)	GNAS mutation (paternal allele)	Present	No	None

Key distinction: In PHP1A, the mutated allele is maternally inherited (GNAS is imprinted — only maternal allele expressed in kidneys). In PPHP, the paternal allele is mutated → AHO features but normal calcium/phosphate metabolism.

Albright's Hereditary Osteodystrophy (AHO)

AHO is the somatic phenotype seen in PHP1A and PPHP:

Brachydactyly — shortened 4th and 5th metacarpals/metatarsals (most characteristic)
Short stature
Obesity
Round facies
Subcutaneous ossifications (ectopic bone)
Developmental delay / intellectual disability
Dental anomalies

AHO and PHP features: brachydactyly, hand X-ray, brain MRI with basal ganglia calcification, and GNAS mutation chromatogram

Clinical image showing brachydactyly (Panel A), metacarpal shortening on X-ray (Panel B), basal ganglia/cerebellar heterotopic calcification on MRI (Panel C), and heterozygous GNAS frameshift mutation on sequencing (Panel D).

Clinical Features of Hypocalcemia in PHP

Neuromuscular irritability: tetany, Chvostek's sign, Trousseau's sign
Perioral and peripheral paresthesias
Carpopedal spasm
Seizures (in severe hypocalcemia)
Prolonged QT interval on ECG
Cataracts (chronic)
Basal ganglia calcification (Fahr's syndrome-like appearance on CT/MRI)

Biochemical Profile

Parameter	PHP	True Hypoparathyroidism
Serum Ca²⁺	Low	Low
Serum PO₄	High	High
PTH	Elevated	Low/undetectable
Urinary cAMP	Low/no rise after PTH	Rises normally
1,25(OH)₂D	Low	Low
Magnesium	Normal	Check (hypoMg → ↓PTH)

Diagnosis

Biochemistry: Hypocalcemia + hyperphosphatemia + elevated PTH (confirms resistance, not deficiency)
Ellsworth-Howard Test (historical): Exogenous PTH infusion → measure urinary cAMP and phosphate
- PHP1: blunted cAMP and phosphate response
- PHP2: normal cAMP but blunted phosphaturia
GNAS molecular genetics: Sequencing for mutations/methylation defects
Imaging: Hand X-ray (metacarpal shortening), brain CT (basal ganglia calcification), DXA (bone density)
TSH/LH/FSH/IGF-1: Screen for multi-hormone resistance in PHP1A

Management

Hypocalcemia

Calcium supplementation: Oral calcium carbonate or citrate (1–3 g elemental calcium/day)
Active vitamin D (calcitriol): 0.25–2 µg/day — critical since 1α-hydroxylation is impaired
- Monitor: serum calcium, urine calcium/creatinine ratio (hypercalciuria risk)
Acute tetany: IV calcium gluconate (10 mL of 10% solution slowly)

Multi-hormone resistance (PHP1A)

Hypothyroidism: Levothyroxine replacement (TSH resistance common)
Growth hormone deficiency: GH therapy if documented (GHRH resistance)
Hypogonadism: Sex hormone replacement if indicated

AHO features

No specific treatment for brachydactyly or short stature beyond GH therapy
Weight management for obesity
Subcutaneous ossifications: surgical excision if symptomatic

Key Pearls

PHP is the only condition where PTH is high yet the patient is hypocalcemic and hyperphosphatemic — distinguish from vitamin D deficiency (phosphate is typically low/normal)
PPHP has AHO features but normal calcium — same GNAS mutation, different parental origin (imprinting)
Always check TSH in PHP1A — subclinical hypothyroidism is common and often overlooked
Hypomagnesemia can mimic PHP by impairing PTH secretion AND causing PTH resistance — always check Mg²⁺ first
Brain CT may show basal ganglia calcification due to chronic hypocalcemia

Sources: Harrison's Principles of Internal Medicine, 21st Edition (pp. 11614, 3188–3190)

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