Parathyroid hormone (serum)
PTH regulates calcium and phosphate homeostasis. During pregnancy, demands for calcium increase, leading to adaptive changes in maternal PTH, PTHrP, vitamin D, and calcium metabolism.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| pg/mL | 8 – 51 | 10 – 15 | 18 – 25 | 9 – 26 |
| ng/L | 8 – 51 | 10 – 15 | 18 – 25 | 9 – 26 |
Pregnancy physiology
- PTH may be low-normal in early pregnancy due to high calcitriol levels.
- PTHrP (placenta & breast) increases and partially replaces PTH function.
- Calcium demand increases as fetal skeletal mineralization accelerates.
- Ionized calcium remains stable despite total calcium dropping from hemodilution.
Causes of low PTH
- Normal pregnancy effect (common in 1st trimester)
- Hypercalcemia (suppresses PTH)
- Vitamin D excess
- Hypomagnesemia
- Autoimmune or postsurgical hypoparathyroidism
- Genetic disorders affecting parathyroids
Causes of elevated PTH
- Secondary hyperparathyroidism (most common in pregnancy): - Vitamin D deficiency - Low calcium intake - Malabsorption
- Primary hyperparathyroidism
- Chronic kidney disease
- Pseudohypoparathyroidism
- Medications: lithium, loop diuretics
Hypercalcemia in pregnancy increases the risk of neonatal hypocalcemia due to suppressed fetal parathyroids.
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.
- Kovacs CS. Maternal mineral and bone metabolism during pregnancy, lactation, and post-weaning recovery. Physiol Rev. 2016.
- Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D.