title: Renal reabsorption of inorganic phosphorus in pregnancy in relation to the calciotropic hormones.
abstract: OBJECTIVE
  To measure renal reabsorption of inorganic phosphorus and the calciotropic hormones in early and late pregnancy.
  
  
  METHODS
  Prospective, cross-sectional study.
  
  
  METHODS
  Endocrine Institute at Assaf Harofeh and E. Wolfson Medical Centers; the Department of Obstetrics and Gynaecology, Sheba Medical Centre and Tel Aviv University.
  
  
  METHODS
  Three groups of healthy women were studied: pregnant women at the end of the first trimester (n = 20), pregnant women at the end of the third, trimester (n = 22), and nonpregnant controls (n = 27).
  
  
  METHODS
  The renal tubular maximal phosphorus reabsorption per decilitre of glomerular filtrate (TmP/GFR) was measured in all women. Circulating levels of intact parathyroid hormone, calcitriol (1,25-dihydroxy vitamin D3) and insulin-like growth factor I were assayed in part of the women (8-11 of each group).
  
  
  RESULTS
  TmP/GFR was elevated in the first trimester group (mean 0.263 mmol/L) compared with controls (95% CI 0.07-0.46, P = 0.003). Third trimester values did not differ from controls. Serum calcitriol in the first trimester group was higher (mean difference 17.68 pg/mL) compared with controls (95% CI 3.89-31.47, P = 0.006) and was higher still (mean difference 20.75 pg/mL) in the third trimester group (95% CI 1.01-40.49, P = 0.042). Serum parathyroid hormone in the first trimester group was lower than in controls or the third trimester group: mean differences were 4.40 pg/mL (95% CI-1.40 to 10.15, P = 0.078) and 8.18 pg/mL (95% CI 0.51-15.85, P = 0.019) respectively. Parathyroid hormone levels correlated negatively to calcitriol levels in the combined control and first trimester groups (r = -0.54, P = 0.022) and negatively to TmP/GFR values in the combined three groups (r = -0.68, P = 0.042). First trimester levels of insulin-like growth factor I were lower than those in controls or in the third trimester: mean differences were 10.24 nmol/L (95% CI 2.05-18.43, P = 0.007) and 13.57 nmol/L (95% CI 4.23-22.91, P = 0.003), respectively.
  
  
  CONCLUSIONS
  The dominant change in mineral metabolism in pregnancy is a rise in calcitriol which most probably is responsible for the relative suppression of parathyroid hormone and thereby for the rise in TmP/GFR in early pregnancy. All the above support the transfer of minerals to the fetus without compromising maternal bone. The significance of circulating insulin-like growth factor I remains unclear.
authors.affiliation: Endocrine Institute, Assaf Harofeh Medical Center, Zrifin, Israel.
authors.forename: M
authors.initials: M
authors.lastname: Weiss
authors.forename: Z
authors.initials: Z
authors.lastname: Eisenstein
authors.forename: Y
authors.initials: Y
authors.lastname: Ramot
authors.forename: S
authors.initials: S
authors.lastname: Lipitz
authors.forename: A
authors.initials: A
authors.lastname: Shulman
authors.forename: Y
authors.initials: Y
authors.lastname: Frenkel
mesh_terms: D000328:Adult; D002117:Calcitriol / Q000097:blood*; D003430:Cross-Sectional Studies; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000032:analysis*; D010281:Parathyroid Hormone / Q000097:blood*; D010758:Phosphorus / Q000378:metabolism*; D011247:Pregnancy / Q000378:metabolism*; D011261:Pregnancy Trimester, First; D011263:Pregnancy Trimester, Third; D011446:Prospective Studies
chemical_list: D010281:Parathyroid Hormone; D010758:Phosphorus; D007334:Insulin-Like Growth Factor I; D002117:Calcitriol