Comprehensive Laboratory Tests for Parathyroid Gland Evaluation
Beyond primary tests like intact parathyroid hormone (iPTH) and serum calcium, several additional laboratory tests are essential for comprehensive evaluation of parathyroid gland function, including 25-hydroxyvitamin D, phosphorus, and specialized PTH assays that can detect biologically active fragments. 1
Core Laboratory Tests for Parathyroid Evaluation
- Intact Parathyroid Hormone (iPTH): The cornerstone test for parathyroid function assessment
- Serum Calcium: Almost always elevated in primary hyperparathyroidism
- Phosphorus: Often low or low-normal in primary hyperparathyroidism due to PTH's phosphaturic effect 1
- 25-hydroxyvitamin D: Essential to exclude vitamin D deficiency as a concomitant cause of secondary hyperparathyroidism 1, 2
- 1,25-dihydroxyvitamin D: Helps assess PTH's effect on vitamin D metabolism 3
Specialized PTH Assays
- Whole/Bioactive PTH Assays: Newer assays that specifically detect the first several amino acids in a 2-site assay, providing more accurate measurement of biologically active PTH 3
- PTH Fragment Analysis: Measures C-terminal fragments (e.g., 7-84 PTH fragment) which may have inhibitory biological activity 3
- Non-(1-84) PTH Measurement: Helps distinguish between biologically active PTH and potentially inhibitory fragments 3
Additional Biochemical Markers
- Alkaline Phosphatase: Increases predictive power when used with PTH levels to assess bone turnover 3
- Fibroblast Growth Factor 23 (FGF23): Important regulator that interacts with PTH in calcium-phosphate homeostasis 3
- Magnesium: Influences PTH secretion and action; deficiency can cause functional hypoparathyroidism 4
Frequency of Testing Based on CKD Stage
| CKD Stage | Calcium & Phosphorus | PTH |
|---|---|---|
| Stage 3 | Every 6-12 months | Every 6-12 months |
| Stage 4 | Every 3-6 months | Every 3-6 months |
| Stage 5 | Every 1-3 months | Every 3 months |
| Dialysis | Monthly | Every 3 months |
Important Considerations and Pitfalls
Vitamin D Status: Vitamin D deficiency can cause secondary hyperparathyroidism and affect PTH levels; 95.3% of individuals with vitamin D deficiency may show elevated PTH 2
Biotin Interference: Can result in falsely high or low PTH results depending on assay design 1
PTH Assay Limitations: Standard intact PTH assays may overestimate biologically active PTH by detecting C-terminal fragments that lack amino acids from the N-terminus 3
Post-translational Modifications: Oxidation and phosphorylation of PTH can affect biological activity and assay results 3
Kidney Function: C-terminal PTH fragments accumulate in kidney disease, potentially affecting test interpretation; their half-life is 5-10 times longer than intact PTH and even longer in kidney disease 3
Dynamic Testing
Calcium Infusion Test: Evaluates PTH suppression in response to elevated calcium; useful in assessing parathyroid gland sensitivity 4
EDTA Infusion Test: Evaluates PTH stimulation in response to lowered calcium; can help correlate with parathyroid gland size 4
By implementing this comprehensive panel of laboratory tests, clinicians can more accurately diagnose parathyroid disorders, distinguish between primary and secondary hyperparathyroidism, and optimize treatment strategies to improve patient outcomes related to morbidity and mortality.