Determinants of plasma PTH and their implication for defining a reference interval

BACKGROUND: To improve the diagnostic sensitivity of PTH measurements, more data on the upper limit of the reference interval for PTH levels were requested at a recent international consensus conference. As PTH levels vary inversely with plasma 25-hydroxyvitamin D (25OHD) levels and as vitamin D insufficiency is widespread, particular attention should be given to the influence of low vitamin D levels on the PTH reference interval. AIM, DESIGN AND METHODS: In a cross-sectional design, including 2316 women aged 17-84, we determined 95% reference interval using a nonparametric approach and studied the effects of potential predictors on plasma PTH levels. RESULTS: PTH was a positive function of age, body weight and BMI and inversely associated with total daily calcium intake, smoking, plasma calcium levels and 25OHD levels, all of which explained 16% of the variability in plasma PTH levels. The threshold value for 25OHD levels below which PTH levels started to rise was 82 nmol/l. Plasma PTH levels varied inversely with the seasonal variations in 25OHD levels. Mean PTH level was 4·1 pmol/l with a reference interval equal to 2·0-8·6 pmol/l. Restricting the population in whom the reference interval was calculated to only women with 25OHD levels above 30 or 100 nmol/l lowered the upper limit of the reference interval to 8·4 and 7·1 pmol/l, respectively. Similar, stratification according to age, body mass index, smoking and calcium intake had only minor impact on the reference interval. CONCLUSION: Indices with known effects on plasma PTH levels have only a minor impact on the upper levels of the normative reference interval in women with intact renal function.