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.