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The relationship between PTH and 25-hydroxy vitamin D early in pregnancy
James E Haddow1, Louis M Neveux, Glenn E Palomaki
1Division of Medical Screening and Special Testing, Department of Pathology and Laboratory Medicine, Women & Infants Hospital/Alpert Medical School of Brown University, Providence, RI, USA. jhaddow@ipmms.org
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 °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.
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