Appropriate Goal Level for 25-Hydroxyvitamin D in Cystic Fibrosis

CHEST Appropriate Goal Level for 25-Hydroxyvitamin D in Cystic Fibrosis Original Research Natalie E. West, MD, MHS; Noah Lechtzin, MD, MHS, FCCP; Christian A. Merlo, MD, MPH; Jason B. Turowski, MD; Marsha E. Davis, MPH; Meghan Z. Ramsay, CRNP; Sharon L. Watts, RN; Shane P. Stenner, MD; and Michael P. Boyle, MD, FCCP CYSTIC FIBROSIS Background: Vitamin D deficiency is common in patients with cystic fibrosis (CF), and guidelines recommend 25-hydroxyvitamin D (25OHD) levels 30 ng/ml. This threshold was selected because serum parathyroid hormone (PTH) rises in healthy individuals when the 25OHD level falls below 30 ng/ml. PTH levels. 50 pg/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 patients with CF by identifying the level below which the risk of PTH. 50 pg/ml begins to increase. Methods: Levels of 25OHD and PTH in 216 individuals with CF were collected prospectively. Individuals with 25OHD, 30 ng/ml were treated with vitamin D2, and levels were reevaluated. Results: Mean 25OHD level was 25.7 12.4 ng/ml, and mean PTH level was 46.7 25.9 pg/ml. In 63% of individuals, 25OHD level was, 30 ng/ml, and in 38.0% it was 20 ng/ml. Low 25OHD levels were significantly associated with elevated PTH levels, with a mean PTH of 53.1 29.8 pg/ml for 25OHD level 0 to 19 ng/ml; 51.1 30.7 pg/ml for 25OHD level 20 to 29 ng/ml; 38.4 16.4 pg/ml for 25OHD level 30 to 39 ng/ml; and 37.2 16.4 pg/ml for 25OHD level 40 ng/ml ( P 5.006). We assessed the sensitivity of different 25OHD thresholds to identify individuals meeting the goal of a PTH level, 50 pg/ml to reduce the risk of bone loss. To obtain 90% sensitivity, a 25OHD level 35 ng/ml was required. Strikingly, 23% of individuals with 25OHD levels 30 to 34 ng/ml still had a PTH level. 50 pg/ml. This decreased to 14% for 25OHD level 35 ng/ml. 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. CHEST 2011; 140(2):469 474 Abbreviations: 1,25OH 2 D 5 1,25-dihydroxyvitamin D; 25OHD 5 25-hydroxyvitamin D; CF 5 cystic fibrosis; CFF 5 Cystic Fibrosis Foundation; PTH 5 parathyroid hormone Cystic fibrosis (CF) is characterized by chronic pulmonary infections, sinus disease, pancreatic insufficiency, male infertility, cystic fibrosis-related diabetes, and osteoporosis. 1 Over the past sev eral decades, median predicted survival has improved from 16 years of age in 1970 to. 37 years currently.2 Decreased bone mineral density and increased risk of fracture are common complications in CF. 1 Multiple studies have reported that 50% to 75% of adults with CF have low bone density, 1 with approximately 20% to 25% having osteoporosis and 40% having osteopenia. 3 Multiple factors contribute to this increased risk of osteoporosis, including malnutrition, vitamin D deficiency, inadequate absorption of calcium, physical inactivity, and glucocorticoid use. 1 Additionally, the prevalence of vitamin D insufficiency in individuals with CF is significantly higher when compared with healthy control subjects, most likely secondary to malabsorption of fat-soluble vitamins resulting from pancreatic insufficiency, poor nutrition, and low exposure to sunlight. 4 Vitamin D is obtained from sunlight by the skin and converted from 7-dehydrocholesterol to cholecalciferol (vitamin D3) or absorbed from the diet www.chestpubs.org CHEST / 140 / 2 / AUGUST, 2011 469

and converted from ergosterol to ergocalciferol (vitamin D2). Both are then converted in the liver to 25-hydroxyvitamin D (25OHD). 25OHD is the major storage form of vitamin D and is measured clinically to assess adequacy of vitamin D stores. 5 25OHD is then converted by the kidney to its active form, 1,25-dihydroxyvitamin D (1,25OH 2 D), which maintains calcium homeostasis. 1 Low vitamin D levels, defined as 25OHD levels, 30 ng/ml, are common in individuals with CF regardless of season or latitude. 1 The threshold of 30 ng/ml was selected because levels below this in individuals without CF lead to a rise in serum parathyroid hormone (PTH). 6 Elevated PTH levels are strongly associated with increased bone resorption and resultant bone loss, particularly PTH levels. 50 pg/ml. 1,6-10 Guidelines from the Cystic Fibrosis Foundation s (CFF s) Consensus Conference on Bone Health recommend that vitamin D2 supplementation be given to maintain 25OHD levels 30 ng/ml. 1 However, the 25OHD threshold at which PTH begins to rise in CF has not been determined. It is unclear whether a goal level of 30 ng/ml is appropriate for CF, particularly given the multiple challenges encountered when attempting to maintain individuals with CF at this level. Therefore, the purpose of this investigation was to determine the appropriate therapeutic 25OHD level in individuals with CF by determining the threshold below which there is an increased risk of elevated PTH levels. 50 pg/ml. Serum 25OHD values were determined at a single Quest Diagnostics laboratory (Teterboro, New Jersey) using liquid chromatography-tandem mass spectroscopy. Serum PTH levels were determined in a single hospital laboratory using two-site sandwich immunoassay PTH on the Elecsys 2010 platform (Roche Diagnostics; Indianapolis, Indiana). Per CFF guidelines, adults at the Johns Hopkins CF Program have 25OHD levels measured annually. 1 Individuals also have annual PTH, 1,25OH 2 D, and calcium levels measured. As described in an earlier publication, 3 individuals with 25OHD levels, 30 ng/ml are routinely treated with vitamin D2 50,000 International Units po daily for 30 days, and 25OHD and PTH levels are reevaluated 2 weeks thereafter. Statistical Analysis Summary statistics were calculated, and data are presented as appropriate by means SD, medians and interquartile range, or proportions. Graphical analyses were performed using scatterplots and histograms. The Shapiro-Wilk W test was used to check for normality. Means were compared using paired and unpaired t tests where appropriate. The Kruskal-Wallis test or analysis of variance was used to compare means of three or more groups. Proportions were compared using x 2 or Fisher exact test. Logistic regression was used to assess the association between 25OHD and PTH. We calculated the sensitivity and specificity of different 25OHD values to predict elevated PTH levels. These calculations were made using a threshold value of PTH. 50 pg/ml, because review of the literature demonstrates increased risk of bone density loss above this level. 8,9,11-13 Analysis was also performed using a definition of. 75 pg/ml as the threshold for elevated PTH level to assure similar results were obtained. Statistical significance was set at P,.05 for all analyses. Analyses were performed using STATA 10.0 (StataCorp; College Station, Texas). Materials and Methods This was a cohort study in which we collected data on all individuals seen at the Johns Hopkins Adult CF Clinic from January 2004 through January 2009 who had 25OHD and PTH (nonfasting) levels drawn. 1,25OH 2 D and calcium levels were also recorded. Individuals with HIV, chronic renal insufficiency, and organ transplant were excluded, as these disease processes are known to affect PTH levels and vitamin D status. This study was approved by the Johns Hopkins University School of Medicine institutional review board (protocol 03-03-21-02e). Manuscript received August 16, 2010; revision accepted January 14, 2011. Affiliations: From the Division of Pulmonary and Critical Care Medicine, Department of Medicine (Drs West, Lechtzin, Merlo, and Boyle and Mss Ramsay and Watts), and the Department of Nutrition (Ms Davis), Johns Hopkins University, Baltimore, MD; the Division of Pulmonary and Critical Care Medicine, Department of Medicine (Dr Turowski), University of Pennsylvania, Philadelphia, PA; and the Department of Medicine (Dr Stenner), Vanderbilt University, Nashville, TN. Funding/Support: The authors have reported to CHEST that no funding was received for this study. Correspondence to: Natalie E. West, MD, MHS, 1830 E Monument St, 5th Floor, Baltimore, MD 21205; e-mail: nwest5@jhmi.edu. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians ( http://www.chestpubs.org/ site/misc/reprints.xhtml ). DOI: 10.1378/chest.10-2114 Results Two hundred sixteen individuals with CF were included in this study, and their demographics are summarized in Table 1. Twenty-one individuals with HIV, chronic renal insufficiency, or lung transplant were excluded. Of the 216 individuals, 136 (63.0%) had 25OHD levels below the currently recommended 30 ng/ml. Fifty-four (25.0%) were vitamin D insufficient (25OHD level, 21-29 ng/ml), and 82 (38.0%) were vitamin D deficient (25OHD level 20 ng/ml). Eighty individuals (37.0%) were vitamin D sufficient (25OHD level 30 ng/ml), with 50 (23.1%) with 25OHD levels 35 ng/ml. The mean ( SD) 25OHD level was 25.7 12.4 ng/ml (normal 30-100 ng/ml). The mean PTH level was 46.7 25.9 pg/ml (normal 10-50 pg/ml). A total of 23.6% of the participants had PTH levels 50 to 74 pg/ml, and 12.5% had levels. 75 pg/ml. No participants had serum calcium levels. 10.7 mg/dl or, 7.2 mg/dl (normal 8.4-10.5 mg/dl). The mean 1,25OH 2 D level was 46.7 16.0 pg/ml (normal 25-66 pg/ml). We found no correlation between 25OHD and 1,25OH 2 D levels ( P 5.40, test of Ho), consistent with prior reports. 7,14 470 Original Research

Table 1 Participant Characteristics (N 5 216) Age, y 29 10 (18-68) Female 107 (49.5) FEV 1, % predicted 62.7 24.3 (14.9-144.7) BMI, kg/m 2 22.2 4.0 (15.0-42.8) delf508/delf508 genotype 100 (46.3) Pancreatic insufficient 182 (84.3) Bone density by DEXA Normal 106 (49.1) Osteopenia 86 (39.8) Osteoporosis 24 (11.1) Mean 25OHD, ng/ml 25.7 12.4 Vitamin D deficient, 20 ng/ml 82 (38.0) Vitamin D insufficient, 21-29 ng/ml 54 (25.0) Vitamin D sufficient 30-34 ng/ml 30 (13.9) 35 ng/ml 50 (23.1) Data are presented as No. (%), mean SD, or mean SD (range). 25OHD 5 25-hydroxyvitamin D; DEXA 5 dual energy x-ray absorptiometry. Relationship Between 25OHD and PTH Levels Figure 1 demonstrates the inverse relationship between 25OHD and PTH levels in the study group. The overall correlation between 25OHD and PTH level was r 5 20.25, P 5.0003. Low vitamin D levels were clearly associated with elevated PTH levels, with mean SD (median) PTH values being 53.1 29.8 (51) pg/ml for 25OHD levels 0 to 19 ng/ml; 51.2 30.7 (42) pg/ml for 25OHD levels 20 to 29 ng/ml; 40.4 18.0 (36) pg/ml for 25OHD levels 30 to 34 ng/ml; and 36.4 15.0 (34.5) pg/ml for 25OHD levels 40 ng/ml ( Fig 2 ). From the standpoint of a caregiver of a patient with CF, the most important question is at what 25OHD level is PTH, 50 pg/ml, where bone resorp- tion and risk of fracture is decreased? We assessed the sensitivity of different 25OHD thresholds in identifying individuals with PTH levels within the ideal range (, 50 pg/ml). The sensitivity of 25OHD levels to predict a PTH level, 50 pg/ml is shown in Table 2. To obtain 90% sensitivity of having a PTH level, 50 pg/ml in our CF study cohort, a goal 25OHD level of 35 ng/ml was required. Using the 2005 CFF guidelines of a target 25OHD level of 30 ng/ml, 1 18.7% of individuals still had a PTH level associated with increased bone loss (. 50 pg/ml). Strikingly, 23% of the individuals with 25OHD levels 30 to 34 ng/ml had a PTH level. 50. This decreased to 14% for 25OHD levels 35. Effect of Vitamin D Supplementation on PTH Levels in CF Of the 216 individuals who had initial measurements of 25OHD and PTH levels, 140 (65%) had subsequent measurements. We specifically analyzed individuals who were initially vitamin D deficient with 25OHD levels 20 ng/ml to determine the effect of vitamin D2 supplementation on their PTH levels. In these 36 individuals, initial mean ( SD) 25OHD level was 15.1 4.2 ng/ml, and the mean PTH level was 52.3 28.0 pg/ml. All were treated with vitamin D2 50,000 IU po daily for 30 days. Mean 25OHD level after supplementation was 48.7 19.3 ng/ml ( P,.001), and mean PTH level decreased significantly to 41.9 20 pg/ml, ( P 5.05). Twenty (56.6%) had PTH levels. 50 pg/ml before treatment, whereas only seven (19.4%) individuals had a PTH level. 50 pg/ml afterward. One important observation, however, was that correcting vitamin D deficiency was not long-lived. Eighteen of the 36 corrected individuals subsequently became vitamin D insufficient again within 1 year. At a mean of 15.1 8.3 months, mean ( SD) 25OHD level decreased from 48.7 19.3 to 18.9 6.5 ng/ml ( P 5.0002). The protocol at our CF clinic is to check 25OHD and PTH levels 2 weeks after supplementation with vitamin D2 as above and then yearly thereafter. Therefore, it is possible deficiency occurred earlier, but the exact time course of becoming vitamin D deficient again could not be calculated. During the study period, maintenance therapy was standard CF vitamins, which included 800 to 1,000 International Units of cholecalciferol (D3) daily. Figure 1. Relationship between 25OHD and PTH levels. Scatterplot of the relationship between all 25OHD and PTH levels in the study group. The overall correlation was r 5 2 0.25, P 5.0003. Low 25OHD levels are associated with elevated PTH levels. 25OHD 5 25-hydroxyvitamin D; PTH 5 parathyroid hormone. Discussion The recognition of osteoporosis and vitamin D deficiency as significant problems in individuals with CF has led to increased efforts to optimize vitamin D www.chestpubs.org CHEST / 140 / 2 / AUGUST, 2011 471

Figure 2. Quartiles of 25OHD levels and correlating PTH levels. Low 25OHD levels are significantly associated with elevated serum PTH levels. Twenty-three percent of the individuals with 25OHD levels 30 to 34 ng/ml had a PTH. 50. This decreased to 14% for 25OHD levels 35. At serum 25OHD levels between 30 and 39 ng/ml, 19% of patients still have PTH levels. 50 pg/ml. Lower and upper hinge of each box plot represents the 25th and 75th percentiles, respectively, with the median value represented as a bold line within each box. The whiskers are drawn to the smallest and largest observations that fall within 1.5 times the interquartile range, with outliers represented as outside of these values. See Figure 1 legend for expansion of abbreviations. and calcium metabolism. These efforts include the 2005 CFF Consensus on Bone Health Guidelines, which recommend that individuals with CF receive vitamin D2 supplementation to maintain their 25OHD level 30 ng/ml. 1 However, this recommendation was based on the relationship between 25OHD and PTH levels in individuals without CF. To our knowledge, our study is the first to specifically examine the relationship between 25OHD and PTH in individuals with CF. It confirms the well-documented high prevalence of vitamin D deficiency in CF but has the key finding that caregivers of patients with CF should likely be aiming to correct 25OHD levels to. 35 ng/ml. Table 2 Sensitivity of 25OHD Levels for Suppressing PTH Levels to, 50 pg/ml 25OHD Cut Point Sensitivity, % 20 49.3 25 69.3 30 81.3 32 85.3 34 88.0 35 90.7 38 92.0 40 93.3 42 96.0 44 96.0 46 98.7 PTH 5 parathyroid hormone. See Table 1 legend for expansion of other abbreviation. Correction to 35 ng/ml results in a greater likelihood of suppressing PTH levels below those associated with increased bone loss. There is currently much discussion of what constitutes a normal 25OHD level, even in healthy individuals. Most experts agree that a 25OHD level 20 ng/ml is consistent with vitamin D deficiency, and a 25OHD level of 21 to 29 ng/ml is considered vitamin D insufficiency. 14 The most commonly accepted range of normal 25OHD level is 32 to 100 ng/ml. 14 However, it is uncertain what the optimal 25OHD level should be, as this range is based on obtaining blood from several hundred healthy volunteers and assigning the normal range as the mean 2 SDs. It is now known that 30% to 50% of the US and European population is vitamin D deficient or insufficient, which may have resulted in skewing of the normal range. 14 One physiologic method to determine optimal 25OHD level is to evaluate its effect on PTH, which increases in response to perceived calcium insufficiency. The most cited study of this relationship concluded that PTH begins a significant increase in value in individuals without CF when 25OHD levels drop to, 30 ng/ml. 6 Yet, there is interindividual variability in this relationship, and similar to our study, 20% of the individuals with 25OHD levels between 30 and 34 ng/ml had PTH levels. 50 pg/ml. 6 Also, as observed in our study, the percentage of patients with PTH. 50 ng/ml decreased to 11% for 25OHD levels 35 to 40 ng/ml, and remained at this level for 25OHD levels. 40 ng/ml. 6 Prevention of elevated PTH levels is of clinical importance because investigators have demonstrated that increased levels of PTH are independently associated with an increased risk of fracture and a faster rate of bone density loss. 6,8,10,15-17 Specifically, once PTH levels exceed 50 pg/ml, there is a significant increase in the risk of fracture. 10 Garnero and colleagues 10 reported an 80% increase in the risk of fractures (relative risk, 1.8; 95% CI, 1.01-3.4) in postmenopausal women whose PTH levels were. 50 pg/ml. All of these observations have led researchers to question if individuals in the normal population would benefit from 25OHD levels. 30 ng/ml. 5,6,18 In a similar fashion, we assessed 25OHD effect on PTH to help determine the most appropriate minimum goal for 25OHD levels in patients with CF. We specifically sought to determine the 25OHD level at which a clinician could have confidence that PTH is, 50 pg/ml, given its known association with increased risk for bone loss. The most important finding of this study is that a goal 25OHD level 35 ng/ml in a patient with CF provides a clinician with more confidence that his or her patient will not have a PTH level in a range 472 Original Research

associated with increased bone loss and risk of fracture. Clinicians can be 90% confident of suppression of PTH levels below 50 pg/ml if 25OHD levels 35 ng/ml are achieved. At 25OHD levels between 30 and 34 ng/ml, approximately 23% of patients with CF will still have PTH levels. 50 pg/ml. These data suggest that future guidelines for treatment of vitamin D deficiency in patients with CF should consider a higher goal 25OHD level. Additionally, results of this study demonstrate that vitamin D2 supplementation can result in correction of elevated PTH levels. There were 82 individuals (37.8%) who initially were vitamin D deficient (25OHD levels 20 ng/ml). Thirty-six of these individuals received sufficient supplementation to improve their 25OHD level 30 ng/ml. This resulted in a statistically significant decrease in PTH levels from 52.3 28.0 pg/ml to 41.9 20.0 pg/ml ( P 5.03). Another lesson from this study is that maintenance of 25OHD levels in the desired range requires close attention and likely an increase in long-term maintenance vitamin D supplementation. In one-half of the individuals whose 25OHD levels were corrected from, 20 to. 30 ng/ml, levels subsequently declined again to, 30 ng/ml the following year, despite ongoing supplementation with at least 800 International Units of vitamin D2 daily. The design of the current study did not allow determination of the exact time course of this decline. This tendency for rapid decline over time may provide further reason to aim for 25OHD levels of 35 ng/ml rather than the current guidelines of 30 ng/ml, as it is clear from previous studies that PTH most rapidly rises when 25OHD levels drop below 30 ng/ml. A total of 84.3% of the patients in our study were pancreatic insufficient, which is similar to the rate found in the CF population nationally. These individuals may require more therapy to maintain vitamin D stores, highlighting the potential importance of recent research suggesting that maintenance therapy with vitamin D3 may be a more effective way of maintaining 25OHD levels in patients with CF. 4 Specifically, many caregivers of patients with CF have begun using daily 2,000 International Units vitamin D3, in addition to vitamin D already found in CF vitamins and diet, to maintain 25OHD levels at goal. Also recently available is 50,000 International Units vitamin D3, which can be given once or twice monthly to help maintain 25OHD levels. Finally, there is a known seasonal variation in 25OHD levels, with levels in the winter and spring more likely to be low. Although 2005 CFF guidelines suggest testing 25OHD levels in the late fall, further research is needed to clarify the optimal monitoring strategy. One potential concern about aiming for higher goal 25OHD levels is the risk of hypervitaminosis D. The highest 25OHD level measured in this study was 99 ng/ml, even with aggressive repletion regimens of vitamin D2 50,000 International Units daily for 30 days. This patient did not demonstrate signs of hypervitaminosis D and had a normal serum calcium level. Vitamin D intoxication is defined as a 25OHD level. 150 ng/ml associated with hypercalcemia, hyperphosphatemia, and hypercalciuria. 5 I t is estimated that 25OHD levels. 150 to 200 ng/ml 5 are required for the development of vitamin D intoxication, an unlikely complication of aiming for a 25OHD level of 35 ng/ml. It is also important to recall that there are pathologic conditions that result in elevated PTH levels independent of vitamin D status. These include primary hyperparathyroidism and renal disease, so the presence of hypercalcemia or PTH levels not consistent with measured 25OHD levels should lead to exploration of other possible contributors. In summary, vitamin D deficiency and decreased bone density are important complications in individuals with CF, and consensus guidelines for optimizing CF bone health have, therefore, been developed. This study suggests that the current recommended guideline of maintaining 25OHD levels 30 ng/ml in patients with CF may still place close to 25% of patients at risk for having elevated PTH levels. Aiming for only a level of 30 ng/ml in patients with CF with its difficult-to-maintain 25OHD levels may also put patients at risk for experiencing the rapid rise in PTH known to occur when 25OHD levels are, 30 ng/ml. Aiming instead to maintain 25OHD levels 35 ng/ml will give the CF clinician greater confidence in reducing PTH levels below the range associated with increased risk of fracture and loss of bone mineral density. Because of the challenge of maintaining 25OHD levels at this level in CF, both close monitoring in individual patients and further research on appropriate chronic vitamin D supplementation are required. Acknowledgments Author contributions: Dr West had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Dr West: contributed to study concept and design; acquisition, analysis, and interpretation of data; statistical analysis; drafting of the manuscript; critical revision of the manuscript for important intellectual content; and administrative, technical, or material support. Dr Lechtzin: contributed to study concept and design; analysis, and interpretation of data; statistical analysis; drafting of the manuscript; critical revision of the manuscript for important intellectual content; and administrative, technical, or material support. Dr Merlo: contributed to study concept and design and critical revision of the manuscript for Dr Turowski: contributed to acquisition of data and critical revision of the manuscript for Ms Davis: contributed to critical revision of the manuscript for www.chestpubs.org CHEST / 140 / 2 / AUGUST, 2011 473

Ms Ramsay: contributed to critical revision of the manuscript for Ms Watts : contributed to critical revision of the manuscript for Dr Stenner: contributed to critical revision of the manuscript for Dr Boyle: contributed to study concept and design; analysis, and interpretation of data; statistical analysis; drafting of the manuscript; critical revision of the manuscript for important intellectual content; administrative, technical, or material support; and study supervision. Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article. Other contributions: We thank Suzanne Jan de Beur, MD, for her insightful commentary in manuscript preparation. The work was performed at Johns Hopkins University.. References 1. Aris RM, Merkel PA, Bachrach LK, et al. Guide to bone health and disease in cystic fibrosis. J Clin Endocrinol Metab. 2005 ;90(3):1888-1896. 2. Cystic Fibrosis Foundation. Annual Data Report to the Center Directors. Bethesda, MD: Cystic Fibrosis Foundation; October 2008. Patient Registry 2007. 3. Boyle MP, Noschese ML, Watts SL, Davis ME, Stenner SE, Lechtzin N. Failure of high-dose ergocalciferol to correct vitamin D deficiency in adults with cystic fibrosis. Am J Respir Crit Care Med. 2005 ;172(2):212-217. 4. Khazai NB, Judd SE, Jeng L, et al. Treatment and prevention of vitamin D insufficiency in cystic fibrosis patients: comparative efficacy of ergocalciferol, cholecalciferol, and UV light. J Clin Endocrinol Metab. 2009 ;94(6):2037-2043. 5. Holick MF. Vitamin D status: measurement, interpretation, and clinical application. Ann Epidemiol. 2009 ; 19 ( 2 ): 73-78. 6. Chapuy MC, Preziosi P, Maamer M, et al. Prevalence of vitamin D insufficiency in an adult normal population. Osteoporos Int. 1997 ;7(5):439-443. 7. Silverberg SJ. Vitamin D deficiency and primary hyperparathyroidism. J Bone Miner Res. 2007 ;22(suppl 2 ):V100-V104. 8. Sahota O, Mundey MK, San P, Godber IM, Lawson N, Hosking DJ. The relationship between vitamin D and parathyroid hormone: calcium homeostasis, bone turnover, and bone mineral density in postmenopausal women with established osteoporosis. Bone. 2004 ;35 (1 ):312-319. 9. Joly D, Drueke TB, Alberti C, et al. Variation in serum and plasma PTH levels in second-generation assays in hemodialysis patients: a cross-sectional study. Am J Kidney Dis. 2008 ; 51 (6 ):987-995. 10. Garnero P, Sornay-Rendu E, Claustrat B, Delmas PD. Biochemical markers of bone turnover, endogenous hormones and the risk of fractures in postmenopausal women: the OFELY study. J Bone Miner Res. 2000 ;15 (8 ):1526-1536. 11. Souberbielle JC, Lawson-Body E, Hammadi B, Sarfati E, Kahan A, Cormier C. The use in clinical practice of parathyroid hormone normative values established in vitamin D- sufficient subjects. J Clin Endocrinol Metab. 2003 ; 88 ( 8 ): 3501-3504. 12. Brown SA, Ontjes DA, Lester GE, et al. Short-term calcitriol administration improves calcium homeostasis in adults with cystic fibrosis. Osteoporos Int. 2003 ;14 (5 ):442-449. 13. Harkness L, Cromer B. Low levels of 25-hydroxy vitamin D are associated with elevated parathyroid hormone in healthy adolescent females. Osteoporos Int. 2005 ;16 (1 ):109-113. 14. Hollis BW. Assessment of vitamin D status and definition of a normal circulating range of 25-hydroxyvitamin D. Curr Opin Endocrinol Diabetes Obes. 2008 ;15 (6 ):489-494. 15. Meunier PJ, Chapuy MC, Arlot ME, Delmas PD, Duboeuf F. Can we stop bone loss and prevent hip fractures in the elderly? Osteoporos Int. 1994 ;4 (suppl 1 ):71-76. 16. Martínez ME, del Campo MT, Sánchez-Cabezudo MJ, et al. Relations between calcidiol serum levels and bone mineral density in postmenopausal women with low bone density. Calcif Tissue Int. 1994 ;55 (4 ):253-256. 17. Rosen CJ, Morrison A, Zhou H, et al. Elderly women in northern New England exhibit seasonal changes in bone mineral density and calciotropic hormones. Bone Miner. 1994 ; 25 ( 2 ): 83-92. 18. Thomas MK, Lloyd-Jones DM, Thadhani RI, et al. Hypovitaminosis D in medical inpatients. N Engl J Med. 1998 ; 338 (12 ):777-783. 474 Original Research