Why Parathyroid Hormone Increases During Pregnancy
Parathyroid hormone (PTH) does NOT increase during pregnancy—it actually decreases by approximately 50% during the first and second trimesters compared to postpartum levels, despite increased maternal calcium demands. 1
The Paradox of Calcium Homeostasis in Pregnancy
PTH Levels Throughout Pregnancy
- PTH levels are significantly decreased during the first and second trimesters compared to non-pregnant women, measuring approximately 50% of postpartum values 1
- By the third trimester, PTH levels begin to normalize but remain lower than postpartum baseline 2, 1
- Ionized calcium remains stable throughout all trimesters despite these hormonal changes 1
The Compensatory Mechanism: Vitamin D Takes Over
The key to understanding this paradox is that 1,25-dihydroxyvitamin D (calcitriol) increases twofold during pregnancy and becomes the primary regulator of calcium homeostasis, not PTH 1. This creates a reciprocal relationship:
- Elevated 1,25-dihydroxyvitamin D suppresses PTH secretion through negative feedback while simultaneously enhancing intestinal calcium absorption 1
- Intestinal calcium transport increases 2.1- to 2.2-fold during pregnancy, even in the complete absence of parathyroid glands, demonstrating that this adaptation is PTH-independent 3
- This vitamin D-mediated increase in calcium absorption is sufficient to meet the 250-300% increase in urinary calcium excretion that occurs during pregnancy 1
Clinical Implications and Monitoring
When PTH Actually Does Increase
The only scenario where PTH appropriately increases during pregnancy is in post-bariatric surgery patients, where monitoring should include:
- Serum 25-hydroxyvitamin D with calcium, phosphate, magnesium, and PTH at least once per trimester 4
- Vitamin D supplementation to maintain levels ≥50 nmol/L with serum PTH within normal limits 4
- Calcium supplementation of 1200-1500 mg daily in divided doses (including dietary intake) to maintain PTH within normal limits 4
Common Pitfall to Avoid
Do not assume elevated PTH is a normal physiologic response to pregnancy—if PTH is elevated, investigate for:
- Primary hyperparathyroidism, which causes complications including hypocalcemic tetany in the newborn 5
- Vitamin D deficiency, which would require correction before PTH can normalize 4
- Malabsorption states (particularly post-bariatric surgery) requiring aggressive supplementation 4
The Fetal Perspective
- The fetus maintains higher calcium levels than the mother through active placental calcium transport 5, 6
- Fetal hypercalcemia suppresses fetal PTH and stimulates fetal calcitonin, creating an environment favorable to skeletal growth 5
- Maternal hypoparathyroidism does not cause fetal calcium deficiency—fetuses from parathyroidectomized mothers remain euparathyroid with normal serum calcium 3