Familial Infantile Hypercalcemia with Suppressed Intact Parathyroid Hormone (PTH)

Familial infantile hypercalcemia (FIH) is a rare, inherited problem where a baby or young child has too much calcium in the blood (hypercalcemia) even though the parathyroid hormone (PTH) level is low or suppressed. Calcium gets too high because the body either cannot break down active vitamin D properly or makes too much active vitamin D when kidney phosphate handling is abnormal. As a result, the gut absorbs extra calcium, the kidneys leak a lot of calcium into urine, and calcium can build up in the kidneys (nephrocalcinosis) or form stones (nephrolithiasis). Children may have vomiting, constipation, poor feeding, dehydration, and slow growth. In many cases, FIH is caused by pathogenic variants in the genes CYP24A1 (Type 1) or SLC34A1 (Type 2). In both types, PTH is appropriately low because the high calcium “turns off” PTH release. New England Journal of Medicine+2Frontiers+2

Familial infantile hypercalcemia (FIH) is a genetic form of high blood calcium that often begins in infancy. Calcium is high in the blood. PTH is low or “suppressed.” The most common cause is a change (variant) in the CYP24A1 gene. This gene makes the enzyme that deactivates vitamin D. When the enzyme is weak, active vitamin D builds up. The gut absorbs extra calcium. Blood calcium rises. The kidneys leak calcium into urine. Kidney stones and nephrocalcinosis can occur. Another cause is a change in SLC34A1, a kidney phosphate transporter. Low phosphate can also push vitamin D to be more active and raise calcium. Adults can present later with kidney stones or pregnancy-related hypercalcemia. PMC+2edm.bioscientifica.com+2

People with CYP24A1-related FIH typically have low or very low 24,25-dihydroxy-vitamin D and a high ratio of 25-OH-D to 24,25-(OH)₂D. This ratio helps confirm the diagnosis, especially when genetic testing is pending. Values ≥50 are highly suggestive of CYP24A1 deficiency. Genetic testing of CYP24A1 (and sometimes SLC34A1) establishes the cause. PMC+2PMC+2

In Type 1 (CYP24A1 deficiency), the enzyme that normally inactivates vitamin D (24-hydroxylase) does not work well. Active vitamin D (1,25-dihydroxyvitamin D) stays high, so calcium absorption increases and blood calcium rises. In Type 2 (SLC34A1 variants), the kidney phosphate transporter NaPi-IIa is impaired, causing phosphate wasting and signals that raise active vitamin D levels; again, calcium rises and PTH stays low. Both types can cause kidney calcium deposits and stones from infancy to adulthood. MedlinePlus+2MedlinePlus+2

Other names

• Idiopathic infantile hypercalcemia (IIH) – historic term used before the genes were discovered; now often split into infantile hypercalcemia type 1 (IH1, CYP24A1) and type 2 (IH2, SLC34A1). MedlinePlus

• CYP24A1 deficiency – another name for type 1 disease. New England Journal of Medicine

• SLC34A1-related infantile hypercalcemia – another name for type 2 disease. Frontiers

• Vitamin D hypersensitivity due to CYP24A1 – emphasizes the strong response to normal vitamin D amounts. Frontiers

Types

1. Type 1: CYP24A1-related familial infantile hypercalcemia
   This type happens when both copies of the CYP24A1 gene have a loss-of-function variant. The enzyme 24-hydroxylase is weak or absent, so the body cannot inactivate active vitamin D, causing high 1,25-(OH)₂D, high calcium, high urine calcium, low PTH, and kidney calcium deposits. Some patients present in infancy; others show kidney stones later in life. Even single-copy (monoallelic) carriers may have milder symptoms under certain triggers. New England Journal of Medicine+2PMC+2

2. Type 2: SLC34A1-related familial infantile hypercalcemia
   This type is due to biallelic variants in SLC34A1, which encodes NaPi-IIa, a key kidney phosphate transporter. Children waste phosphate in urine, which stimulates active vitamin D production, leading to high calcium, low PTH, and nephrocalcinosis. Some people with one variant may still develop kidney stones or hypercalciuria. Frontiers+1

Note about SLC34A3 (NaPi-IIc): Variants in SLC34A3 cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH), which also features low phosphate and high 1,25-(OH)₂D, but it is not the classic FIH; PTH is typically low-normal, and rickets is common. It is useful mainly as a differential diagnosis when a child has low phosphate and high urine calcium. PMC+1

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Causes

Below, “cause” means the root genetic reason and also the common triggers that worsen or unmask the high-calcium state in babies/children with these genes.

1. Biallelic CYP24A1 loss-of-function variants (Type 1). The enzyme that inactivates vitamin D is impaired. New England Journal of Medicine

2. Biallelic SLC34A1 variants (Type 2). Kidney phosphate wasting raises 1,25-(OH)₂D. Frontiers

3. Monoallelic CYP24A1 or SLC34A1 variants plus stressors. A single variant can still contribute to symptoms in some people. PMC

4. High vitamin D supplementation in pregnancy (maternal). Can unmask disease after birth in a genetically susceptible infant. Frontiers

5. High vitamin D intake in the infant (drops, fortified formula). Extra substrate pushes calcium higher in Type 1 and Type 2. Frontiers

6. High sun exposure in sensitive infants. More vitamin D synthesis may worsen hypercalcemia. Frontiers

7. Low dietary phosphate. Worsens phosphate depletion signaling in Type 2. Frontiers

8. Dehydration or poor fluid intake. Concentrates calcium in urine and promotes kidney damage. OUP Academic

9. Intercurrent illness with poor feeding. Less fluid and phosphate intake can aggravate labs and symptoms. OUP Academic

10. Thiazide diuretics exposure. These reduce urinary calcium excretion and can raise serum calcium. (Important as a medication trigger to avoid in this disorder.) OUP Academic

11. Calcium-rich formula or supplements. More absorbed calcium adds to hypercalcemia. OUP Academic

12. Prolonged immobilization (rare in infants, relevant later). Increases bone calcium release. OUP Academic

13. Genetic background modifying vitamin D metabolism. Some families show variable severity with the same variants. Karger Publishers

14. Concurrent SLC34A3 variant (rare). May worsen kidney calcifications in a person who already has SLC34A1 disease. PMC

15. Maternal vitamin D loading close to delivery. Transiently high vitamin D status in infant. Frontiers

16. Excessive cod-liver oil or multivitamin use. Hidden vitamin D sources. Frontiers

17. Inadequate monitoring after starting routine vitamin D drops. Unrecognized sensitivity in infants with CYP24A1 variants. New England Journal of Medicine

18. High dietary calcium without balancing phosphate. Tilts toward higher calcium absorption. Frontiers

19. Use of calcium-alkali products (e.g., antacids) in older children. Adds to calcium load. OUP Academic

20. Delayed diagnosis with ongoing vitamin D and calcium intake. Prolongs the hypercalcemic state and kidney exposure. OUP Academic

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Common symptoms and signs

1. Poor feeding and frequent vomiting in infancy. Babies may refuse feeds and spit up often. chikd.org

2. Constipation. Hard stools are common with high calcium. chikd.org

3. Irritability and sleepiness. Babies can seem fussy or unusually drowsy. chikd.org

4. Failure to thrive (slow weight gain). Growth may lag behind age expectations. chikd.org

5. Dehydration (dry mouth, sunken eyes, low tears). High urine losses worsen this. OUP Academic

6. Polyuria and polydipsia (peeing and drinking a lot). Parents notice heavy diapers. OUP Academic

7. Muscle weakness and reduced reflexes. Hypercalcemia dampens nerve-muscle function. ScienceDirect

8. Poor growth in length (stunting) over time if not treated. OUP Academic

9. Abdominal pain or colic. Can be related to stones or constipation. OUP Academic

10. Kidney stones and nephrocalcinosis (calcium in kidney tissue). May be found on ultrasound. OUP Academic

11. Blood pressure may be high in some children with kidney involvement. OUP Academic

12. Recurrent urinary tract infections if stones obstruct or irritate the tract. OUP Academic

13. Poor appetite and weight loss in older infants/children. chikd.org

14. Bone pain in older children (less common in infants). OUP Academic

15. No symptoms at times. Some children are found by screening or after a kidney ultrasound. OUP Academic

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Diagnostic tests

A) Physical examination

1. General growth check (weight, length/height, head circumference). Doctors look for poor weight gain or stunting that suggests long-standing metabolic stress. Serial measurements help track recovery after treatment. OUP Academic

2. Hydration assessment (skin turgor, mucous membranes, tears). Hypercalcemia causes polyuria; babies can get dehydrated quickly. The exam guides urgent fluid replacement. OUP Academic

3. Blood pressure measurement. Kidney calcifications and salt-water balance changes can increase blood pressure; early detection prevents complications. OUP Academic

4. Abdominal and flank palpation. Tenderness may suggest stones; a careful exam plus history of colicky pain directs imaging. OUP Academic

5. Neuromuscular tone and reflex check. Hypercalcemia can cause hypotonia and reduced tendon reflexes; this supports the lab picture. ScienceDirect

B) Bedside/manual assessments

6. Strict intake–output charting (including diaper weight). Helps quantify polyuria and tailor fluids. It also shows improvement when calcium normalizes. OUP Academic

7. Orthostatic vital signs (when age-appropriate). Dizziness or tachycardia with position change hints at volume depletion from high urine output. OUP Academic

8. Diet and supplement inventory. A careful “manual” review of all vitamin D drops, multivitamins, formula, fortified foods, and calcium products identifies triggers to stop. Frontiers

C) Laboratory & pathological tests

9. Serum total and ionized calcium. Confirms hypercalcemia; ionized calcium shows the physiologically active level. In FIH, calcium is high. ScienceDirect

10. Intact PTH. In FIH, PTH is low/suppressed because high calcium turns it off—this separates FIH from primary hyperparathyroidism. ScienceDirect

11. 25-hydroxyvitamin D [25(OH)D]. Often normal or high-normal. Important to interpret with other vitamin D markers. Frontiers

12. 1,25-dihydroxyvitamin D [1,25(OH)₂D]. Characteristically elevated in both Type 1 and Type 2, driving intestinal calcium absorption. Frontiers

13. 24,25-dihydroxyvitamin D and the 25(OH)D:24,25(OH)₂D ratio. In CYP24A1 deficiency, 24,25-(OH)₂D is low, and the ratio is high, which is highly suggestive of Type 1. New England Journal of Medicine

14. Serum phosphate and alkaline phosphatase (ALP). Type 2 often shows low phosphate from kidney losses; ALP may be normal/low relative to age versus rickets. Frontiers

15. Urine calcium/creatinine ratio (spot) or 24-h urinary calcium. Hypercalciuria is common and tracks with kidney risk. OUP Academic

16. Fractional excretion of phosphate (or tubular maximum for phosphate). High urinary phosphate wasting suggests SLC34A1 disease (Type 2). Frontiers

17. Serum creatinine/eGFR and electrolytes. Monitors kidney function and safety of therapy; chronic nephrocalcinosis can reduce kidney function. OUP Academic

18. Genetic testing for CYP24A1 and SLC34A1 (± panels including SLC34A3). Confirms the diagnosis, clarifies the type, and guides family counseling. Frontiers+1

D) Electrodiagnostic tests

19. Electrocardiogram (ECG). Hypercalcemia can cause shortened QT interval and other changes; ECG is quick and non-invasive. ScienceDirect

20. Holter monitoring (only if symptoms such as palpitations). Used selectively in older children if rhythm concerns persist during episodes of hypercalcemia. ScienceDirect

E) Imaging tests

21. Renal ultrasound (first-line). Detects nephrocalcinosis and stones without radiation and is the key imaging test in infants and children. OUP Academic

22. Kidneys–ureters–bladder (KUB) X-ray. May show radio-opaque stones if ultrasound is unclear or to follow known stones. Use sparingly due to radiation. OUP Academic

23. Low-dose non-contrast CT (reserved cases). Sensitive for small stones if ultrasound and X-ray are inconclusive and symptoms are strong. Balance benefit vs radiation. OUP Academic

24. Renal Doppler ultrasound (selected). Assesses renal blood flow when there is concern for complications from heavy calcification. OUP Academic

25. Bone radiographs (selected). If growth delay or bone pain is prominent, X-rays can look for secondary bone effects; rickets suggests alternative diagnoses like HHRH. PMC

Non-pharmacological treatments

1. Stop all vitamin D supplements (including “multivitamins”).
   Purpose: reduce vitamin D activation that is driving hypercalcemia.
   Mechanism: less substrate for 1α-hydroxylase; lowers 1,25-(OH)₂D over time. Frontiers

2. Minimize UV-B sunlight exposure (sunscreen, shade).
   Purpose: lower skin vitamin D production.
   Mechanism: cuts cholecalciferol synthesis that feeds the pathway. MDPI

3. Generous oral fluids (unless restricted by your clinician).
   Purpose: prevent dehydration and help kidneys excrete calcium.
   Mechanism: increases urine flow and calcium excretion; reduces stone risk. Bioscientifica

4. Tailored dietary calcium (avoid extremes).
   Purpose: do not overload calcium but keep enough for bone, especially in children.
   Mechanism: avoiding very high calcium intake reduces absorbed calcium; chronic severe restriction is discouraged because it can raise oxalate absorption and stone risk. OUP Academic

5. Avoid thiazide diuretics unless a specialist advises otherwise.
   Purpose: prevent worsening of blood calcium.
   Mechanism: thiazides decrease urinary calcium and can raise serum calcium—caution in FIH. Frontiers

6. Avoid prolonged immobilization.
   Purpose: slow bone calcium release.
   Mechanism: weight-bearing and movement reduce bone resorption linked to inactivity. MDPI

7. Limit high-vitamin-D fortified foods during flares (e.g., fortified milks, some cereals).
   Purpose: reduce vitamin D load.
   Mechanism: less dietary vitamin D lowers active metabolite generation. Frontiers

8. Kidney stone prevention diet basics (if stones or nephrocalcinosis).
   Purpose: lower stone formation.
   Mechanism: high fluids, moderate dietary calcium, lower sodium, limit very high oxalate foods, and increase citrate help deter calcium stone formation. Frontiers

9. Sodium restriction (especially with hypercalciuria).
   Purpose: cut urinary calcium.
   Mechanism: less sodium intake decreases calcium excretion in the proximal tubule. Frontiers

10. Citrate boosting (lemons/limes) or medical citrate if prescribed.
    Purpose: raise urinary citrate (a stone inhibitor).
    Mechanism: citrate binds urinary calcium and reduces crystal growth. Frontiers

11. Education on “hidden” vitamin D sources.
    Purpose: prevent accidental excess.
    Mechanism: label reading avoids OTCs and fortified products that raise vitamin D. MDPI

12. Pregnancy planning with endocrinology & obstetrics.
    Purpose: reduce maternal and fetal risks.
    Mechanism: strict vitamin D avoidance and tailored calcium/fluids; some meds are limited in pregnancy. Bioscientifica

13. Genetic counseling for families.
    Purpose: understand inheritance and testing of relatives.
    Mechanism: autosomal recessive patterns and variable expressivity guide screening. BioMed Central

14. Routine renal ultrasound follow-up.
    Purpose: detect stones/nephrocalcinosis early.
    Mechanism: imaging tracks kidney mineral deposition over time. PMC

15. ECG if calcium is markedly high or symptoms occur.
    Purpose: assess for short QT and rhythm risks.
    Mechanism: hypercalcemia shortens QT interval; ECG shows changes promptly. OUP Academic

16. Acute care plan: IV saline first in significant/symptomatic hypercalcemia.
    Purpose: stabilize quickly.
    Mechanism: volume expansion increases renal calcium excretion. (Therapy delivered in hospital.) Bioscientifica

17. Sun-safe lifestyle (protective clothing).
    Purpose: keep vitamin D production consistently low.
    Mechanism: blocks UV-B to reduce cholecalciferol formation. MDPI

18. Avoid unnecessary high-dose vitamin A or retinoids.
    Purpose: some retinoids can affect bone resorption/mineral metabolism.
    Mechanism: reduces potential add-on effects on calcium handling. Frontiers

19. Prompt treatment of dehydration from GI illness.
    Purpose: prevent calcium concentration from spiking.
    Mechanism: maintains kidney perfusion and calciuresis. Bioscientifica

20. Specialist follow-up (endocrinology/nephrology).
    Purpose: adjust diet and medicines as the child grows or as an adult’s situation changes.
    Mechanism: periodic labs, imaging, and medication review. PMC

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Medicines

Important: FIH is not malignancy. Many hypercalcemia drugs are approved for other causes (e.g., cancer). In FIH, clinicians use them off-label to lower calcium or protect kidneys. I cite the FDA label for accurate dosing/safety details, and clinical literature for the FIH-specific rationale.

1) Isotonic IV saline (hospital).
Class: volume expander (not an FDA “drug product” label like others here).
Dose/Time: titrated to clinical status.
Purpose/Mechanism: expand volume, increase kidney calcium excretion. Side effects: fluid overload in heart/kidney disease. (Acute guideline reference). Bioscientifica

2) Furosemide (Lasix®) – loop diuretic (used after rehydration).
Typical dosing (label context): individualized IV dosing; labels outline diuretic indications; in hypercalcemia, given after fluids to promote calciuresis.
Purpose/Mechanism: blocks NKCC2 in loop of Henle; increases urinary calcium excretion.
Key safety: risk of dehydration, electrolyte loss, ototoxicity; careful monitoring. FDA Access Data+1

3) Calcitonin-salmon (Miacalcin®) – rapid, temporary
Class: calcitonin receptor agonist.
Dose/Time: acute hypercalcemia regimens are in clinical protocols; the FDA label details formulations and warnings.
Purpose/Mechanism: quickly reduces osteoclastic bone resorption; effect wanes (tachyphylaxis).
Safety: nausea, flushing; malignancy signal in some long-term datasets on nasal spray labeling. FDA Access Data+1

4) Pamidronate disodium (Aredia®) – IV bisphosphonate
Class: anti-resorptive; osteoclast inhibitor.
Dose/Time (label for hypercalcemia of malignancy): 60–90 mg IV over 2–24 h; severe cases 90 mg once.
Purpose/Mechanism: inhibits osteoclast activity → lowers calcium; case reports support benefit in severe infant FIH.
Safety: flu-like symptoms, renal toxicity (dose/infusion-rate related). FDA Access Data+1

5) Zoledronic acid (Zometa®) – IV bisphosphonate
Class: anti-resorptive.
Dose/Time (label contexts): used IV; labels detail renal cautions and hypocalcemia risk; HCM dosing commonly 4 mg IV.
Purpose/Mechanism: potent osteoclast inhibition; long duration.
Safety: renal impairment risk; correct hypocalcemia before dosing; osteonecrosis of jaw risk. FDA Access Data+1

6) Denosumab (Xgeva®) – RANKL inhibitor
Class: monoclonal antibody blocking RANKL.
Dose/Time (label for HCM): 120 mg SC every 4 weeks with extra doses on days 8 and 15 in month 1.
Purpose/Mechanism: shuts down osteoclast formation; useful if bisphosphonates are contraindicated.
Safety: hypocalcemia (especially in kidney disease), osteonecrosis of jaw; needs calcium/vitamin D per label to prevent hypocalcemia (clinicians balance this in FIH). FDA Access Data

7) Glucocorticoids (e.g., prednisone/prednisolone)
Class: anti-inflammatory; lowers calcitriol in some vitamin-D–mediated states.
Purpose in FIH: evidence suggests poor effect in CYP24A1 deficiency; may help other calcitriol-mediated hypercalcemia.
Safety: many dose-dependent effects; see label. Frontiers+1

8) Rifampin (Rifadin®) – enzyme inducer (off-label here)
Class: antibiotic; induces CYP3A4 which enhances alternative vitamin-D catabolism.
Use: case reports/series show lowered 1,25-(OH)₂D and calcium in some CYP24A1 patients; variable response.
Safety: hepatotoxicity, drug interactions; see FDA label. OUP Academic+1

9) Fluconazole (Diflucan®) – azole antifungal (off-label here)
Class: inhibits cytochrome P450 enzymes; reduces 1α-hydroxylase activity.
Use: case reports describe lowering calcitriol and calcium in CYP24A1.
Safety: hepatic effects and QT issues; see FDA label. Frontiers+1

10) Ketoconazole (Nizoral®) – azole antifungal (off-label here, rarely used now)
Class: stronger P450 inhibitor; historically used to suppress calcitriol; limited by toxicity.
Safety: boxed warnings for hepatotoxicity; many drug interactions; generally avoided if alternatives exist. Frontiers+1

11) Potassium citrate (Urocit-K®) – stone prevention
Class: urinary alkalinizer; citrate supplement.
Purpose: raises urinary citrate and lowers stone risk in hypercalciuria/nephrolithiasis.
Dosing/Safety: extended-release tablets with defined mEq strengths; monitor potassium and renal function. FDA Access Data

12) Oral phosphate (K-Phos Neutral®)
Class: phosphate supplement.
Purpose: in SLC34A1-related forms with phosphate wasting, carefully dosed phosphate may help correct low phosphate and dampen vitamin-D activation (specialist guided).
Safety: GI upset, electrolyte shifts; see product information. DailyMed

13) Potassium phosphates (IV) – in hospital if severe hypophosphatemia
Class: parenteral phosphate.
Purpose: corrects low phosphate that can worsen vitamin-D activation (SLC34A1 contexts).
Safety: dosing limits and ECG monitoring for potassium per label. FDA Access Data

14) Cinacalcet – calcimimetic (rare, selected reports)
Class: increases Ca-sensing receptor sensitivity → lowers PTH.
Note: PTH is already low in classic FIH; one report suggests benefit in a special CYP24A1 context; this is not routine. Specialist use only. OUP Academic

15) Dialysis (modality, not a drug) when life-threatening and refractory
Purpose/Mechanism: removes calcium directly when medical therapy fails, especially with renal failure.
Evidence: case reports and reviews in refractory hypercalcemic crises. PMC

16) General acute pathway (guideline backdrop):
Hydration → rapid agent (calcitonin) → antiresorptive (bisphosphonate or denosumab), with cause-specific therapy and close monitoring. Endocrine Society

If you’d like, I can expand this list with more drug monographs, but these are the most relevant to FIH care and its complications.

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Dietary molecular supplements

These are adjuncts for stone risk and renal protection, not cures for FIH. Always clear supplements with your clinician, especially for children and during pregnancy.

1. Citrate (medical potassium citrate, prescribed) – raises urinary citrate; binds calcium; lowers crystal growth. Dose is individualized; monitor potassium. FDA Access Data

2. Phosphate (oral K-Phos Neutral, prescribed) – in SLC34A1-related phosphate wasting, careful phosphate can reduce 1,25-(OH)₂D production by normal physiology. Dosed with labs. DailyMed

3. High-citrate foods (lemon/lime juice added to water) – supports urinary citrate; non-drug support for stone prevention. Frontiers

4. Adequate dietary calcium (not very high, not very low) – protects bones and helps bind oxalate in the gut; extreme restriction is discouraged. OUP Academic

5. Lower sodium intake – reduces urinary calcium loss and stone risk. Frontiers

6. Limit very high-oxalate foods if stones are an issue (spinach, nuts, beets) while keeping normal calcium to bind oxalate. Frontiers

7. High fluid intake (main “supplement” is water) – increases urine volume to keep calcium diluted. Frontiers

8. Avoid over-the-counter vitamin D – prevents fueling the underlying problem. MDPI

9. Avoid “bone health” megadose combos with hidden vitamin D or very high calcium; review labels carefully. MDPI

10. Discuss any herbal products (many contain vitamin D analogs or interact with rifampin/azoles). Your clinician will screen for interactions. FDA Access Data+1

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Immunity booster / regenerative / stem-cell drugs

I can’t list FDA-approved immune-booster or regenerative/stem-cell drugs for FIH because none are approved for this purpose, and unapproved stem-cell products can be dangerous. The FDA warns that stem-cell therapies are largely unapproved outside specific indications. For FIH, care focuses on vitamin-D metabolism, hydration, kidney protection, and (select cases) antiresorptives. Safer, evidence-based options are above. If you have a specific product in mind, I can review its FDA status and safety. Endocrine Society

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Surgeries

1. Ureteroscopy with laser lithotripsy – a scope via the urethra and ureter breaks stones and removes fragments; done for symptomatic ureteral stones. Frontiers

2. Ureteral stent placement – a small tube placed to bypass obstruction and allow urine to flow until definitive stone treatment. Frontiers

3. Percutaneous nephrolithotomy (PCNL) – a tiny back incision to access kidney and remove large or complex stones.

4. Shock-wave lithotripsy (SWL) – external shock waves fragment some stones; selection depends on stone size and location.

5. Renal replacement therapy / transplant (rare, end-stage kidney disease) – used only if chronic damage progresses despite prevention.

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Preventive tips

• No vitamin D supplements; minimize UV-B exposure.

• Stay well-hydrated every day.

• Moderate dietary calcium; avoid both very high and very low.

• Lower salt intake to reduce urinary calcium.

• Keep active to avoid bone resorption from immobilization.

• Avoid thiazides unless a specialist prescribes them.

• Review all OTCs for hidden vitamin D.

• Regular kidney checks (urine tests, ultrasound).

• Follow individualized pregnancy plans if applicable.

• Keep specialist follow-ups (endocrine/nephrology).

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When to see a doctor

Seek urgent care for vomiting, confusion, severe weakness, dehydration, chest pain, or palpitations, especially if known high calcium—these can be signs of severe hypercalcemia and heart rhythm changes like a short QT. Contact your clinician promptly for new flank pain, blood in urine, fever with stones, recurrent urinary symptoms, or if pregnancy is possible. Routine visits are needed for lab checks, kidney ultrasound, and medication adjustments.

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What to eat and what to avoid

Eat/Drink: plenty of water; fruits/citrus to boost citrate; normal (not low) dietary calcium in meals; low-salt foods; balanced diet.
Avoid/Limit: vitamin-D-fortified products and supplements; very high-salt foods; very high-oxalate foods if you form stones; thiazides unless directed; unnecessary immobilization.

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FAQs

1. Why is PTH low if calcium is high?
   Because high calcium and high active vitamin D tell the parathyroid glands to “shut off,” so PTH falls.

2. Is this the same as primary hyperparathyroidism?
   No. In FIH, PTH is suppressed, not high. The driver is vitamin-D overactivity from CYP24A1 or phosphate transport defects.

3. What test confirms CYP24A1 deficiency?
   A high 25-OH-D : 24,25-(OH)₂D ratio (often ≥50) and/or CYP24A1 gene variants on genetic testing.

4. Can adults be diagnosed later?
   Yes—some present with kidney stones or during pregnancy.

5. Will glucocorticoids fix it?
   Usually not in CYP24A1 deficiency; they’re often ineffective.

6. Are azoles (fluconazole/ketoconazole) cures?
   No. They may lower calcitriol and calcium in some cases; ketoconazole has important safety limits.

7. What about rifampin?
   It can speed vitamin-D breakdown via CYP3A4; responses vary; monitor liver and interactions.

8. Do I need a very low-calcium diet?
   No. Prolonged severe restriction can increase oxalate absorption and stone risk; use moderation under specialist guidance.

9. Why avoid thiazides?
   They can raise blood calcium; not a first-line choice in FIH.

10. Can stones be prevented?
    High fluids, lower sodium, normal calcium intake, and more citrate help reduce stones.

11. Is denosumab allowed?
    It’s approved for hypercalcemia of malignancy; in FIH it’s an off-label option when needed, with careful monitoring.

12. Are there stem-cell treatments?
    No approved stem-cell or “regenerative” drugs for FIH. Focus is on vitamin-D pathway control and kidney protection.

13. What ECG change happens with high calcium?
    Short QT interval is classic; severe cases can cause arrhythmias.

14. Can pregnancy worsen it?
    Yes—vitamin-D physiology changes in pregnancy can unmask or worsen hypercalcemia; plan closely with specialists.

15. Will my child outgrow this?
    The genetic tendency remains, but severity varies across life. Good habits and follow-up reduce complications.

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Last Updated: October 07, 2025.

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