Pseudohypoparathyroidism: Labs, Type 1A vs 1B & PPHP

Pseudohypoparathyroidism (PHP) is end-organ resistance to PTH: calcium is low and phosphate is high, as in hypoparathyroidism, but PTH is high because the parathyroid glands work and the kidney does not respond. The block sits in the renal proximal tubule, where the PTH receptor signals through Gsα, encoded by the imprinted GNAS gene, and where mainly the maternal allele is expressed. So an inactivating GNAS mutation inherited from the mother causes PHP type 1A (Albright hereditary osteodystrophy plus resistance to PTH, TSH and other Gs-coupled hormones), the same mutation from the father causes pseudopseudohypoparathyroidism (the body habitus with normal labs), and PHP type 1B is mostly isolated PTH resistance from a GNAS methylation (imprinting) defect.

The lab pattern and its look-alikes

The one-line tell: PTH normally makes the kidney dump phosphate, so high PTH with high phosphate and a normal GFR means PTH resistance. The 2018 international consensus defines PTH resistance as hypocalcemia, hyperphosphatemia and elevated PTH without vitamin D deficiency and with normal magnesium and renal function, and advises suspecting it whenever PTH is at or above the upper limit of normal with high phosphate, even before calcium falls. PTH and phosphate rise first; hypocalcemia can follow up to about 4.5 years later. Calcitriol is low or normal and urine calcium is low. The same high-PTH, high-phosphate picture with a low GFR is renal osteodystrophy.

Mechanism: why the kidney fails but bone does not

• Signaling: PTH binds the PTH/PTHrP receptor (PTH1R), a Gs-coupled receptor: Gsα → adenylyl cyclase → cAMP → protein kinase A.
• Proximal tubule: that cAMP signal makes phosphate spill into the urine and switches on 1α-hydroxylase. Without it, phosphate is retained, calcitriol is low or normal, calcium falls and PTH climbs.
• Imprinting: GNAS is expressed from both alleles in most tissues but mainly from the maternal allele in the renal proximal tubule, thyroid, pituitary and ovary, the tissues that become resistant.
• Distal tubule still responds, so calcium reabsorption is preserved: urine calcium is low and hypercalciuria is rare.
• Bone still responds, so chronically high PTH can cause bone resorption, rickets-like changes and even brown tumors.
• Other Gs-coupled hormones affected in PHP1A: TSH, GHRH, gonadotropins and calcitonin.

Step 1 contrast: McCune-Albright syndrome is the mirror image, an activating GNAS mutation that arises after conception (mosaic, not inherited) and keeps adenylate cyclase switched on: polyostotic fibrous dysplasia, "coast of Maine" café-au-lait spots and endocrine overactivity such as precocious puberty.

The types at a glance

• PHP1B genetics: 15–20% of cases are familial, autosomal dominant through the maternal line, usually from a small STX16 deletion that removes a methylation control element. Most cases are sporadic, and 8–10% of those carry paternal uniparental isodisomy of 20q: both copies come from the father, so there is no maternal imprint (the mechanism is laid out in uniparental disomy).
• PHP1C has the full PHP1A picture with normal Gsα activity in laboratory assays; the consensus treats it as a PHP1A variant.
• PHP type 2: exogenous PTH raises urinary cAMP normally but fails to cause phosphaturia; the cause is unknown (a defect downstream of cAMP, or possibly vitamin D deficiency).
• Progressive osseous heteroplasia: also paternal GNAS mutations, with ossification that spreads deep into connective tissue.

Imprinting: the parent-of-origin rule

Because the paternal GNAS allele is largely silent in the proximal tubule and other hormone-responsive tissues:

• Mutation on the maternal allele → those tissues have almost no working Gsα → PTH and TSH resistance plus AHO = PHP1A.
• Mutation on the paternal allele → the healthy maternal allele covers the kidney and thyroid → no hormone resistance, but AHO still develops because tissues such as bone use both alleles → PPHP.
• Transmission: an affected parent passes the mutation to half of their children. A mother's mutation gives the child PHP1A, a father's gives PPHP (or progressive osseous heteroplasia), whatever the parent's own diagnosis.

Memory hook: Mom's mutation Makes hormone resistance; Pop's gives the Physique only. PHP1A has AHO; PHP1B is mostly Biochemical.

Clinical picture

• Hypocalcemia is the main presenting problem: perioral and fingertip tingling, muscle cramps, carpopedal spasm, Chvostek and Trousseau signs, prolonged QT and seizures. Growth spurts and calcium or vitamin D deficiency can unmask it, and diagnosis often waits until puberty (PHP1A) or adolescence and adulthood (PHP1B).
• AHO: the major criteria are brachydactyly type E (premature fusion of the growth plates) and short adult stature; supporting features are a stocky build, round face and ectopic, often subcutaneous, ossifications. The fifth, fourth and third metacarpals are shortened most, so a clenched fist shows dimples where the fourth and fifth knuckles should be (the "knuckle, knuckle, dimple, dimple" sign). Brachydactyly is not specific, though: Turner syndrome can shorten the metacarpals too.
• Ossification vs calcification: AHO's subcutaneous nodules are true bone from Gsα-deficient mesenchymal cells and do not depend on calcium or phosphate levels. By contrast, basal ganglia calcification (Fahr syndrome) comes from chronic hypocalcemia with hyperphosphatemia, so it has not been described in PPHP; cataracts form the same way, from calcium-phosphate deposits in the eye.
• PHP1A extras: a mildly high TSH that can be mistaken for congenital hypothyroidism, obesity from infancy, GH deficiency, hypogonadism and cognitive impairment of variable degree (about 30% have normal cognition).

Diagnosis

1. Labs: calcium, phosphate, PTH, creatinine, magnesium and 25-hydroxyvitamin D, plus TSH; repeat them, because values fluctuate early.
2. Molecular confirmation: GNAS sequencing (PHP1A, PPHP) or GNAS methylation analysis (PHP1B).
3. Ellsworth-Howard test (exam classic): after a PTH infusion, urinary cAMP and phosphate stay flat in type 1, while cAMP rises without phosphaturia in type 2. The consensus calls it unnecessary outside research.
4. Hand radiographs: shortened metacarpals and advanced bone age.

Treatment

Per the 2018 consensus:

• Calcitriol (or alfacalcidol), with or without calcium, is the mainstay for chronic hypocalcemia. Do not use PTH or PTH analogs, since the receptor pathway is the problem.
• Targets: calcium and phosphate in the normal range, with PTH brought down toward the upper part of the reference range but not suppressed. That residual PTH keeps the distal tubule reabsorbing calcium and prevents hypercalciuria, so calcium can be targeted higher than in true hypoparathyroidism, where the goal is low-normal.
• Start early: active vitamin D can be considered even without hypocalcemia when PTH is more than twice the upper limit of normal or rising with hyperphosphatemia.
• Also: keep 25-hydroxyvitamin D normal, give IV calcium for severe symptomatic hypocalcemia, check calcium, phosphate and PTH about every 6 months, and image the kidneys for nephrocalcinosis at transition to adult care.
• Other hormones: levothyroxine for TSH resistance; growth hormone for documented GH deficiency.

Practice question

Vignette. A 10-year-old girl has muscle cramps and carpal spasm after soccer practice. Newborn screening showed a mildly elevated TSH, treated with levothyroxine, and she has been obese since infancy. Height is at the 3rd percentile. She has a round face and a hard subcutaneous nodule on her forearm, and when she makes a fist the fourth and fifth knuckles are replaced by dimples. Calcium is 7.3 mg/dL, phosphate 6.8 mg/dL, PTH 290 pg/mL (reference 10–65), with normal creatinine, magnesium and 25-hydroxyvitamin D. Her mother is short with the same hands but has normal calcium, phosphate and PTH. The daughter's GNAS mutation is identical to her mother's. Which mechanism best explains why only the daughter has abnormal labs?

• A. Anticipation
• B. Genomic imprinting
• C. Incomplete penetrance
• D. Mitochondrial heteroplasmy
• E. Paternal uniparental disomy of 20q
• F. Variable expressivity

Answer: B, genomic imprinting. The daughter inherited the mutation from her mother, so it sits on her maternal allele, the one her proximal tubule and thyroid rely on: PTH and TSH resistance plus AHO, which is PHP1A. In the mother, the same mutation sits on her paternal allele (inherited from her father or arisen there), so her healthy maternal allele protects the kidney and she has pseudopseudohypoparathyroidism.

Why not the others?

• Anticipation is earlier and more severe disease in successive generations from expanding trinucleotide repeats; GNAS defects in PHP are point mutations, deletions or methylation changes.
• Incomplete penetrance means a carrier shows no phenotype at all, but the mother clearly has AHO.
• Mitochondrial heteroplasmy does not apply: GNAS is a nuclear gene on chromosome 20q13.
• Paternal uniparental disomy of 20q causes some sporadic PHP1B through loss of the maternal methylation imprint; this family shares a coding mutation and has AHO.
• Variable expressivity describes differing severity of the same phenotype; here a predictable parent-of-origin effect decides who gets hormone resistance. The difference between the two terms is summarized in penetrance vs expressivity.

Next step for the daughter: calcitriol with calcium, aiming for normal calcium and a PTH near the upper limit of normal, and screening for GH deficiency.

Key takeaways

• PHP = PTH resistance: low calcium, high phosphate, high PTH, with normal GFR, magnesium and vitamin D. Hypoparathyroidism has the same calcium and phosphate but a low PTH.
• GNAS encodes Gsα, the link from PTH1R (and TSH, GHRH and gonadotropin receptors) to cAMP.
• Imprinting: the proximal tubule and thyroid mainly use the maternal allele. Maternal mutation → PHP1A (AHO plus multihormone resistance); paternal mutation → PPHP (AHO, normal labs).
• PHP1B: GNAS methylation defect, mostly isolated PTH resistance, usually without AHO.
• AHO: short stature, round face, stocky build, short fourth and fifth metacarpals, subcutaneous ossifications.
• Treatment: calcitriol plus calcium, keeping PTH near the upper normal limit; levothyroxine and GH as needed; never PTH.

Sources

• Diagnosis and management of pseudohypoparathyroidism and related disorders: first international Consensus Statement — Nature Reviews Endocrinology (2018)
• Pseudohypoparathyroidism — MedlinePlus Medical Encyclopedia
• Pseudohypoparathyroidism — NORD Rare Disease Database
• GNAS gene — MedlinePlus Genetics
• McCune-Albright syndrome — MedlinePlus Genetics
• Hypocalcemia — Merck Manual Professional Edition (2025)
• Hypomagnesemia — Merck Manual Professional Edition (2025)
• Hyperphosphatemia — Merck Manual Professional Edition (2025)
• Hypercalcemia — Merck Manual Professional Edition (2025)
• A Novel GNAS Mutation in a Patient with Ia Pseudohypoparathyroidism (iPPSD2) Phenotype — Genes, MDPI (2023)
• Albright hereditary osteodystrophy — Radiopaedia