Parathyroid Hormone Changes in Normal Pregnancy
Parathyroid hormone (PTH) levels typically decrease or remain low-normal during pregnancy, particularly in the first and second trimesters, rather than increasing as a normal maternal adaptation. 1, 2
Normal Physiologic Pattern
The evidence consistently demonstrates that intact PTH levels follow a specific pattern during pregnancy:
- First and second trimesters: Serum intact PTH concentrations are significantly decreased compared to non-pregnant women 1, 2
- Third trimester: PTH levels may normalize but generally remain within or below the normal non-pregnant range 2
- Mean PTH levels: Studies using modern intact PTH assays show pregnant women have mean PTH of 14.4 ± 6.3 ng/L compared to 24.8 ± 9.0 ng/L in non-pregnant women (P < 0.001) 1
Why the Confusion Exists
Older studies using traditional radioimmunoassays (RIAs) incorrectly suggested "physiological hyperparathyroidism of pregnancy" because these assays detected inactive PTH fragments rather than biologically active intact PTH. 1 Modern immunoradiometric assays for intact PTH have definitively shown this concept to be incorrect.
Mechanism of PTH Suppression
The decrease in PTH during pregnancy appears to be a secondary response to changes in calcium homeostasis:
- Ionized calcium levels decrease significantly between 21-25 weeks' gestation (from 3.81 to 3.63 mg/dl, P < 0.01) 3
- Despite this decrease in ionized calcium, PTH remains suppressed or only modestly elevated, suggesting other factors (likely placental calcium transport and increased calcitriol production) maintain calcium homeostasis 3
- Total serum calcium is lower in normal pregnancy, but ionized calcium remains within normal limits 4
Clinical Implications
When PTH is elevated during pregnancy, this is NOT a normal adaptation and warrants investigation for:
- Primary hyperparathyroidism: The most common pathologic cause (80% due to parathyroid adenoma), which increases maternal and fetal morbidity 4, 5
- Vitamin D deficiency: Should be corrected first, as this is the most common cause of secondary PTH elevation 6
- Malabsorption states: Particularly in women with prior bariatric surgery 7, 6
Monitoring Recommendations
In high-risk populations (e.g., post-bariatric surgery), PTH should be monitored alongside calcium, phosphate, magnesium, and 25-hydroxyvitamin D every trimester or every six months. 7, 6 The goal is to maintain PTH within the normal reference range through adequate vitamin D (≥50 nmol/L) and calcium supplementation (1,200-1,500 mg daily). 6
Key Pitfall to Avoid
Do not dismiss elevated PTH as a "normal pregnancy change." Any elevation of PTH during pregnancy should prompt evaluation for underlying pathology, particularly when serum calcium is elevated above 2.75 mmol/L, as this requires urgent intervention to prevent maternal complications (pancreatitis, hypercalcemic crisis) and fetal complications (prematurity, neonatal hypocalcemia). 5