Chest. 2011 Mar 10.
West NE, Lechtzin N, Merlo CA, Turowski JB, Davis ME, Ramsay MZ, Watts SL, Stenner SP, Boyle MP.
Johns Hopkins University, Department of Medicine, Division of Pulmonary and Critical Care Medicine. nwest5 at jhmi.edu.
BACKGROUND: Vitamin D deficiency is common in Cystic Fibrosis (CF), and guidelines recommend 25-hydroxyvitamin D (25OHD) levels ?30ng/ml. This threshold was selected because parathyroid hormone (PTH) rises in healthy individuals when 25OHD falls below 30ng/ml. PTH levels >50pg/ml are associated with an increased risk of bone loss. However, the relationship between 25OHD and PTH has not been studied in CF. We sought to determine the appropriate goal 25OHD level in CF by identifying the level below which the risk of PTH >50 begins to increase.
METHODS: 25OHD and PTH levels in 216 individuals with CF were collected prospectively. Individuals with 25OHD <30ng/ml were treated with vitamin D2, and levels were reevaluated.
RESULTS: Mean 25OHD was 25.7±12.4ng/ml, and mean PTH was 46.7±25.9pg/ml. In 63.0%, 25OHD was <30ng/ml, and in 38.0% it was ?20ng/ml. Low 25OHD levels were significantly associated with elevated PTH levels, with a mean PTH of 53.1±29.8pg/ml for 25OHD 0-19ng/ml; 51.1±30.7pg/ml for 25OHD 20-29ng/ml; 38.4±16.4pg/ml for 25OHD 30-39ng/ml; and 37.2±16.4pg/ml for 25OHD ?40ng/ml (p-value 0.006). We assessed the sensitivity of different 25OHD thresholds to identify individuals meeting the goal of a PTH <50pg/ml to reduce the risk of bone loss.
To obtain 90% sensitivity, a 25OHD ?35 ng/ml was required. Strikingly, 23% of individuals with 25OHD 30-34ng/ml still had a PTH>50. This decreased to 14% for 25OHD ?35.
CONCLUSIONS: Inadequate serum 25OHD levels are common in adults with CF and are associated with elevated PTH levels. Aiming to maintain 25-OHD levels ?35 ng/ml in individuals with CF decreases the risk of having a PTH level associated with secondary hyperparathyroidism and bone loss.