RENAL REABSORPTION OF INORGANIC PHOSPHORUS IN PREGNANCY IN RELATION TO THE CALCIOTROPIC HORMONES

Objective To measure renal reabsorption of inorganic phosphorus and th e calciotropic hormones in early and late pregnancy. Design Prospectiv e, cross-sectional study. Setting Endocrine Institute at Assaf Harofeh and E. Wolfson Medical Centers; the Department of Obstetrics and Gyna ecology, Sheba Medical Centre and Tel Aviv University. Population Thre e groups of healthy women were studied: pregnant women at the end of t he first trimester (n = 20), pregnant women at the end of the third tr imester (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-di hydroxy vitamin D-3) and insulin-like growth factor I were assayed in part of the women (8-11 of each group). Results TmP/GFR was elevated i n 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 highe r (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 parath yroid 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 = 0.0 19) 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 insuli n-like growth factor I were lower than those in controls or in the thi rd 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), respectivel y. Conclusions The dominant change in mineral metabolism in pregnancy is a rise in calcitriol which most probably is responsible for the rel ative suppression of parathyroid hormone and thereby for the rise in T mP/GFR in early pregnancy. All the above support the transfer of miner als to the fetus without compromising maternal bone. The significance of circulating insulin-like growth factor I remains unclear.