The relationship between PTH and 25-hydroxy vitamin D early in pregnancy

Objective Measure serum PTH and 25(OH)D in a cross-sectional sample of pregnant women at 11th through 13th weeks' gestation to examine vitamin D status and consider implications. Design Observational: we retrieved residual sera stored at) -20 degrees C after routine first trimester Down's syndrome screening, distributed over 12 months. Patients 430 African American women and 586 Caucasian women. Measurements PTH and 25-hydroxy vitamin D [25(OH)D] immunoassays. Results PTH medians were: 1 33 pmol/l (African American women); 1.20 pmol/l (Caucasian women) (t = 0.43, P = 0.7). Concentrations were highest in winter and decreased significantly in spring, fall, and summer. There was a direct PTH/weight relationship in Caucasian (t = 3.12, P < 0.002), but not African American women (t = 1.34, P = 0.18). Median 25(OH)D concentrations were 47.5 nmol/l (African American women) and 65 nmol/l (Caucasian women) (t = 13.7, P < 0.001). Concentrations were lowest in winter and rose significantly in spring, fall, and summer. Reciprocal 25(OH)D/weight relationships existed for both racial groups (t = -4.31 P < 0.001; t = 4.54, P < 0.001, respectively). Among 68 Caucasian women who smoked, median PTH and 25(OH)D concentrations were somewhat lower (P = ns). In separate regression models with PTH and 25(OH)D [dependent variables] and season, weight and smoking [independent variables], the only qualifying interactive term was in the Caucasian PTH model (season* 1/weight). A regression model applied to adjusted scatter plots of PTH vs 25(OH)D indicated a weak relationship. Conclusions The PTH/25(OH)D relationship is weaker during early pregnancy than in non-pregnant adults, making it unreliable for estimating vitamin D sufficiency. A suitable reference point for sufficiency might be the maternal 25(OH)D level considered sufficient for adequate transfer to neonates.