What labs should be assessed when evaluating parathyroid (Parathyroid Hormone) conditions?

Laboratory Assessment for Parathyroid Conditions

When evaluating parathyroid conditions, serum levels of calcium, phosphorus, intact parathyroid hormone (PTH), and alkaline phosphatase activity should be measured as the core laboratory panel. 1

Core Laboratory Panel

Primary Tests

• Intact Parathyroid Hormone (PTH) - The central hormone in calcium homeostasis
• Serum Calcium - Total serum calcium
• Ionized Calcium - More sensitive than total calcium for detecting calcium abnormalities
• Phosphorus - Inversely related to PTH levels
• Alkaline Phosphatase - Marker of bone turnover

Secondary Tests

• 25-hydroxyvitamin D - To assess vitamin D status which affects PTH secretion
• Albumin - For correct interpretation of total calcium levels
• Creatinine/eGFR - To assess kidney function which affects PTH metabolism

Interpretation Considerations

PTH Measurement

• PTH is measured using different generations of immunoassays:

  • Second-generation assays (intact PTH assays) - Detect both full-length PTH and some fragments
  • Third-generation assays - More specific for biologically active PTH 1

• Important considerations:

  • PTH assays vary significantly between laboratories (up to 5-fold differences) 2
  • Results should be interpreted in relation to the specific assay used
  • Sequential measurements should use the same assay for proper trend analysis

Calcium Assessment

• Ionized calcium >6.0 mg/dL has 91.3% association with positive parathyroid imaging 3
• Normal calcium with elevated PTH may indicate early primary hyperparathyroidism or secondary hyperparathyroidism

Phosphate Levels

• Phosphate <3.0 mg/dL has 70.7% association with positive parathyroid imaging 3
• Inverse relationship with PTH levels is diagnostically important

Testing Algorithm Based on Suspected Condition

For Primary Hyperparathyroidism

1. Measure serum calcium, ionized calcium, PTH, phosphate
2. If calcium is elevated with inappropriately normal or high PTH → diagnostic of primary hyperparathyroidism
3. If calcium is normal but PTH is elevated → consider oral calcium loading test to confirm diagnosis 4

For Secondary Hyperparathyroidism

1. Measure serum calcium, phosphate, PTH, alkaline phosphatase, 25-hydroxyvitamin D, and creatinine
2. In CKD patients, frequency of testing should follow this schedule 1:
   • GFR 30-59 ml/min/1.73m² (CKD Stage 3): At least annually
   • GFR <30 ml/min/1.73m² (CKD Stages 4-5): At least twice yearly
   • More frequent monitoring if receiving treatment for abnormalities

Common Pitfalls to Avoid

1. Relying on absolute PTH cutoff values - Guidelines should avoid absolute cutoffs due to assay variability 1

2. Ignoring vitamin D status - Low vitamin D can cause secondary hyperparathyroidism even with normal calcium

3. Not considering kidney function - CKD affects PTH metabolism and interpretation of results

4. Using different PTH assays for sequential measurements - This can lead to misinterpretation of trends

5. Overlooking alkaline phosphatase - The predictive power of PTH levels is increased by concomitant consideration of alkaline phosphatase levels 1

Special Considerations

• In bariatric surgery patients, PTH may be more sensitive than calcium in detecting deficiency 1

• In CKD patients, target ranges for mineral markers are difficult to achieve - only about 12.8% of dialysis patients have all markers within target ranges 5

• During thyroid or parathyroid surgery, rapid PTH measurements can assess surgical success, with third-generation assays showing faster response than second-generation assays 1