Renal reabsorption of inorganic phosphorus in pregnancy in relation to the calciotropic hormones

Objective To measure renal reabsorption of inorganic phosphorus and the calciotropic hormones in early Design Prospective, cross-sectional study. Setting Endocrine Institute at Assaf Harofeh and E. Wolfson Medical Centers; the Deparment of Obstetrics and Gynaecology, Sheba Medical Centre and Tel Aviv University. Population 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 and main outcome measures 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 fiom 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 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 2.05–1 8.43, 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 and late pregnancy. 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) unclear.