Determinants of Plasma Parathyroid Hormone Levels in Young Women

While the effects of calcium, phosphorus intake, and vitamin D on parathyroid hormone (PTH) have been well studied, less is known about other factors that impact PTH. Our goal was to delineate associations between demographic, dietary, and plasma factors and PTH. We conducted a cross-sectional study of intact PTH among 1,288 nonblack women in the Nurses Health Study II aged 33–53 with BMI <30 kg/m² and eGFR ≥60 ml/min/1.73 m². Median PTH was 30.7 pg/ml. After adjusting for 25-hydroxyvitamin D and other factors, PTH was 4.1 pg/ml lower (95% CI −7.7 to −0.5) in women who smoked 1–14 cigarettes/day and 6.4 pg/ml lower (95% CI −11.2 to −1.7) in women who smoked >15 cigarettes/day compared to nonsmokers. After multivariate adjustment, women whose BMI was 27–29 kg/m² had PTH levels 2.0 pg/ml higher (95% CI 0.2–3.9) compared to BMI of 21–22 kg/m² and women in the highest quartile of plasma phosphorus had PTH levels 4.1 pg/ml lower (95% CI −5.8 to −2.4) than women in the lowest quartile. Higher vitamin A intake was independently associated with lower PTH, whereas lower calcium intake, lower plasma calcium, lower plasma 25-hydroxyvitamin D, and winter blood draw were associated with higher PTH. Intakes of phosphorus, animal protein, magnesium, alcohol, and caffeine were not associated with PTH. Factors not classically associated with calcium–phosphorus metabolism impact PTH. Additional research is needed to elucidate the mechanisms whereby smoking, vitamin A, and phosphorus affect PTH and to examine how body size and season may affect PTH independent of 25(OH)D.