Placental impermeability to parathormone 125I in the ewe

Placental permeability and metabolism of parathyroid hormone (PTH) 125I was studied in 5 gravid ewes during the last trimester. The mean plasma half-life was 38 +/- 0.8 and 22.6 +/- 2.7 min in the fetus and ewe, respectively. There was minimal placental transfer of PTH 125I in either the fetal to maternal and maternal to fetal directions. Fetal calcium homeostasis is probably not affected directly by changes in maternal serum PTH levels, and maternal calcium homeostasis probably is not affected directly by changes in fetal serum PTH concentrations.