What is the most likely diagnosis for an adult with elevated intact parathyroid hormone (~88 pg/mL), normal serum calcium (9.1 mg/dL), sufficient 25‑hydroxyvitamin D (≈55 ng/mL), and normal phosphorus (4 mg/dL)?

Normocalcemic Secondary Hyperparathyroidism Due to Vitamin D Insufficiency

Your laboratory values indicate normocalcemic secondary hyperparathyroidism, most likely driven by vitamin D insufficiency (55 ng/mL is sufficient but suboptimal for PTH suppression), and you should supplement with cholecalciferol or ergocalciferol to achieve 25-OH vitamin D levels ≥30 ng/mL while monitoring calcium and phosphorus monthly. 1

Diagnostic Interpretation

Your biochemical profile shows:

• PTH 88 pg/mL (mildly elevated)
• Calcium 9.1 mg/dL (normal: 8.6–10.3 mg/dL) 2
• Vitamin D 55 ng/mL (sufficient, >30 ng/mL) 1
• Phosphorus 4 mg/dL (normal)

This constellation—elevated PTH with normal calcium—defines secondary hyperparathyroidism, not primary hyperparathyroidism. 1 The normal calcium excludes primary hyperparathyroidism, which requires hypercalcemia or at minimum inappropriately normal PTH in the setting of elevated calcium. 2, 3

Why Vitamin D at 55 ng/mL Still Permits PTH Elevation

Although your vitamin D level exceeds the 30 ng/mL threshold used to exclude deficiency, PTH reference values are approximately 20% lower in vitamin D–replete individuals compared to those with unknown or borderline status. 1 In other words, a PTH of 88 pg/mL may reflect residual secondary stimulation in someone whose vitamin D, while "sufficient," has not yet optimally suppressed parathyroid secretion. 1

Rule Out Chronic Kidney Disease

The first mandatory step is to assess kidney function with eGFR, because PTH rises early in chronic kidney disease—often before calcium or phosphorus abnormalities appear. 1 If your eGFR is ≥60 mL/min/1.73 m², CKD is excluded. 1 If eGFR is 30–59 mL/min/1.73 m² (CKD stage G3), you have CKD-related secondary hyperparathyroidism and the management algorithm shifts. 1

Treatment Algorithm

Step 1: Confirm No CKD and Optimize Vitamin D

• Measure eGFR to exclude chronic kidney disease. 1
• Supplement with cholecalciferol (vitamin D₃) or ergocalciferol (vitamin D₂) to achieve and maintain 25-OH vitamin D ≥30 ng/mL. 1, 4 Even though your current level is 55 ng/mL, continued supplementation with a maintenance dose (typically 1,000–2,000 IU daily) ensures sustained adequacy. 1
• Do not use calcitriol or active vitamin D analogs (alfacalcidol, doxercalciferol, paricalcitol) unless you have advanced CKD (stage G4–G5) with severe, progressive hyperparathyroidism (PTH persistently >300 pg/mL). 1, 4 Active vitamin D increases intestinal calcium absorption and raises the risk of hypercalcemia and adynamic bone disease. 5, 1

Step 2: Assess Dietary Calcium and Phosphorus Intake

• Ensure adequate dietary calcium intake (1,000–1,200 mg/day for adults). 1, 2 Low calcium intake can drive secondary hyperparathyroidism even when vitamin D is sufficient. 1
• Total elemental calcium intake (diet plus supplements) should not exceed 2,000 mg/day. 5, 2
• If hyperphosphatemia were present (it is not in your case), dietary phosphate restriction would be the first intervention. 1 Your phosphorus of 4 mg/dL is normal, so no phosphate restriction is needed. 1

Step 3: Recheck PTH After Optimization

• Repeat PTH measurement in 3 months after optimizing vitamin D and calcium intake. 1, 4 PTH exhibits approximately 20% biological variability in healthy individuals, so a single measurement is insufficient to confirm true persistent elevation. 1
• If PTH remains >110 pg/mL (the upper target for CKD stage G3) after correction of reversible factors, this defines persistent secondary hyperparathyroidism. 1

Monitoring Schedule

• Measure serum calcium and phosphorus monthly for the first 3 months, then every 3 months thereafter. 1, 4
• Measure PTH every 3 months for 6 months, then every 3–6 months. 1, 4
• Discontinue all vitamin D therapy immediately if serum calcium exceeds 10.2 mg/dL during supplementation. 5, 2

Critical Pitfalls to Avoid

Do Not Diagnose Primary Hyperparathyroidism

Primary hyperparathyroidism requires hypercalcemia (corrected calcium >10.2 mg/dL) or at minimum inappropriately normal PTH in the setting of elevated calcium. 2, 3 Your calcium of 9.1 mg/dL is solidly normal, excluding this diagnosis. 2

Do Not Suppress PTH to the Normal Range in CKD

If you have CKD stage G3b (eGFR 30–44 mL/min/1.73 m²), do not aim to suppress PTH to the normal range (<65 pg/mL), as mild hyperparathyroidism is protective against adynamic bone disease. 1 The target PTH for CKD G3 is 35–70 pg/mL; for CKD G4, 70–110 pg/mL. 5, 1

Do Not Start Active Vitamin D Without Clear Indication

Routine prescription of calcitriol or analogs is not advised for CKD G3a–G5 patients not on dialysis. 1, 4 Reserve active vitamin D for severe, progressive hyperparathyroidism (PTH persistently >300 pg/mL with upward trend) in CKD G4–G5. 1

Recognize Age-Related GFR Decline

In elderly individuals, the normal age-related decline in GFR is the most frequent cause of elevated PTH despite normal calcium. 1 If you are older, even a "normal" eGFR of 50–59 mL/min/1.73 m² represents CKD stage G3a and explains the PTH elevation. 1

When to Consider Further Workup

• If PTH remains >110 pg/mL after 6 months of optimized vitamin D and calcium intake, and eGFR is >60 mL/min/1.73 m², consider normocalcemic primary hyperparathyroidism—a rare entity defined by persistently elevated PTH with consistently normal albumin-corrected calcium after exclusion of all secondary causes. 6, 7, 8
• Measure ionized calcium if total calcium remains normal but PTH stays elevated, because ionized calcium is elevated in 95% of normocalcemic primary hyperparathyroidism cases when total calcium is normal. 7
• Referral to endocrinology is appropriate if PTH remains >110 pg/mL after correction of reversible factors and ionized calcium is elevated. 1, 4