Is parathyroid hormone (PTH) increased during pregnancy in a healthy woman?

Parathyroid Hormone Levels During Pregnancy

Parathyroid hormone (PTH) is typically decreased or remains in the low-normal range during the first and second trimesters of pregnancy in healthy women, then may rise toward normal levels in the third trimester. 1

Normal Physiological Changes in PTH During Pregnancy

First and Second Trimesters

• Serum intact PTH concentrations are significantly decreased compared to non-pregnant women during the first and second trimesters of pregnancy. 1
• Nephrogenous cyclic AMP excretion, an index of parathyroid function, is also decreased during these early pregnancy stages, consistent with the suppressed PTH levels. 1

Third Trimester

• PTH levels tend to normalize by the third trimester, with concentrations no longer significantly different from non-pregnant women. 1
• Nephrogenous cyclic AMP excretion returns to the normal range during the third trimester, reflecting the recovery of parathyroid function. 1

Mechanism Behind PTH Suppression

The physiological suppression of PTH during early pregnancy likely occurs because:

• Pregnancy poses a significant challenge to maternal calcium homeostasis, requiring adaptive hormonal changes. 1
• The placenta produces PTH-related protein (PTHrP), which helps maintain calcium balance and may contribute to the relative suppression of maternal PTH. 2
• Calcium transport from mother to fetus creates unique metabolic demands that alter the normal calcium-PTH regulatory axis. 2

Clinical Implications

Monitoring Calcium and PTH in Pregnancy

• When monitoring women after bariatric surgery during pregnancy, serum 25-hydroxyvitamin D should be checked along with calcium, phosphate, magnesium, and PTH every 6 months. 3
• Vitamin D supplementation should maintain concentrations of 50 nmol/L or greater with serum PTH within normal limits. 3
• Calcium should be added to ongoing vitamin D supplementation as needed to maintain PTH within normal limits. 3

Pathological Conditions

• Primary hyperparathyroidism (PHPT) during pregnancy is characterized by elevated serum calcium with an inappropriately increased PTH level—the opposite of normal pregnancy physiology. 4
• PHPT is rare during pregnancy (representing only 0.03% of women of reproductive age) but carries significant maternal and fetal risks if calcium exceeds 2.75 mmol/L (11 mg/dL). 5, 6

Important Caveats

• The normal suppression of PTH in early pregnancy means that "normal" PTH levels in a pregnant woman with hypercalcemia are actually inappropriate and suggest primary hyperparathyroidism. 4
• Hypercalcemic crisis can develop after delivery when calcium transport to the fetus is abruptly disrupted, particularly in women with undiagnosed PHPT. 2
• Excessive PTHrP production from the placenta or mammary glands can rarely cause non-malignant hypercalcemia during pregnancy, puerperium, or lactation, even without underlying parathyroid disease. 2