From the Guidelines
The primary lab test for evaluating parathyroid function is the parathyroid hormone (PTH) level, which should be measured alongside serum calcium levels for proper interpretation. A complete parathyroid evaluation typically includes serum calcium, intact PTH, phosphorus, vitamin D (25-hydroxyvitamin D), and kidney function tests (BUN and creatinine) 1. When hyperparathyroidism is suspected, ionized calcium provides a more accurate assessment than total calcium. Additional tests may include 24-hour urine calcium to distinguish between types of hyperparathyroidism, bone density scans to assess bone health, and occasionally parathyroid imaging studies like sestamibi scans or ultrasound if surgical intervention is being considered 1.
These tests work together because parathyroid glands regulate calcium homeostasis, with PTH increasing blood calcium when levels drop by enhancing bone resorption, increasing intestinal calcium absorption, and reducing renal calcium excretion. Abnormal results help diagnose conditions like primary hyperparathyroidism (elevated PTH and calcium), secondary hyperparathyroidism (elevated PTH with normal or low calcium), or hypoparathyroidism (low PTH and calcium) 1. It's essential to note that PTH assays differ in their ability to measure biologically active PTH and PTH fragments, which can affect clinical decisions 1.
Key laboratory tests for evaluating parathyroid function include:
- Serum calcium
- Intact PTH
- Phosphorus
- Vitamin D (25-hydroxyvitamin D)
- Kidney function tests (BUN and creatinine)
- 24-hour urine calcium (in some cases)
- Bone density scans (in some cases)
- Parathyroid imaging studies (in some cases, such as sestamibi scans or ultrasound) 1.
In clinical practice, it's crucial to consider the limitations and differences between PTH assays and their potential impact on diagnosis and treatment decisions 1.
From the Research
Laboratory Tests for Parathyroid Function
The following laboratory tests are used to evaluate parathyroid gland function:
- Serum calcium levels: to assess for hypercalcemia or hypocalcemia 2, 3, 4, 5, 6
- Parathyroid hormone (PTH) levels: to assess for elevated or inappropriately normal levels 2, 3, 4, 5, 6
- Phosphate levels: to assess for abnormalities in phosphate regulation 3, 5, 6
- 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D levels: to assess for vitamin D deficiency or abnormalities in vitamin D regulation 3, 6
- Calcium-to-creatinine clearance ratio: to assess for abnormalities in renal calcium clearance 3
- Serum magnesium and creatinine levels: to assess for abnormalities in magnesium and renal function 4
- Bone mineral density tests: to assess for osteoporosis or osteopenia 2, 4
Diagnostic Criteria
The diagnosis of primary hyperparathyroidism is confirmed by demonstrating persistent hypercalcemia (or normal-high serum calcium levels) in the presence of inappropriately normal or elevated serum PTH levels 3, 6. The calcium-to-phosphorous (Ca/P) ratio is also a useful tool in the diagnosis of primary hyperparathyroidism, with a cutoff of 2.71 (or 3.5 if Ca and P are expressed in mg/dL) having a sensitivity and specificity of 86% and 87%, respectively 5.
Differential Diagnosis
The differential diagnosis of primary hyperparathyroidism includes:
- Tumoral hypercalcemia
- Granulomatous disease (e.g. sarcoidosis, tuberculosis)
- Inadequate intake of 1alpha-hydroxyvitamin D or calcitriol
- Vitamin D or A intoxication
- Lithium intake
- Endocrinopathies (e.g. hyperthyroidism, Addison's disease)
- Treatment with thiazides 3