FREE AND BIOAVAILABLE 25-HYDROXYVITAMIN D LEVELS IN PATIENTS WITH PRIMARY HYPERPARATHYROIDISM

Unraveling the Paradox of Vitamin D Status in Primary Hyperparathyroidism: An Incidental Finding or an Unexpected Consequence?

Suboptimal vitamin D status is commonly observed in primary hyperparathyroidism but is rarely considered in management decisions. The present study aimed to bring additional insights on vitamin D status in primary hyperparathyroidism patients, particularly those presenting with the normocalcemic phenotype. A retrospective study was conducted on 53 confirmed primary hyperparathyroidism patients, stratified into hypercalcemic and normocalcemic groups, hospitalized at the "Pius Brînzeu" Emergency Clinical Country Hospital in Timișoara, Romania. Patients presenting with the normocalcemic phenotype had similar target-organ involvement compared to their counterparts. In this subgroup, 25 hydroxyvitamin D showed an inverse correlation with serum calcium (p = 0.048), and regression analysis identified iPTH and 25OH vitamin D as significant predictors of calcium levels (p < 0.0001; R2 = 0.571). Adenoma volume showed a significant negative correlation with 25OH vitamin D levels (p = 0.021; r = -0.61) but was later found as insignificant after confounder analysis. Postoperative measurements of 25OH vitamin D levels confirmed increasing levels after parathyroidectomy. Our findings highlight a complex relationship between PTH and vitamin D in primary hyperparathyroidism, especially in the often-underdiagnosed normocalcemic phenotype. The inverse correlation between vitamin D and calcium suggests altered homeostasis, rather than true deficiency.

When and How to Evaluate Vitamin D Status? A Viewpoint from the Belgian Bone Club

Low serum vitamin D levels have been associated with a variety of health conditions which has led the medical community but also the general population to evaluate vitamin D status quite liberally. Nevertheless, there remain questions about the efficacy and cost-effectiveness of such a broad and untargeted approach. This review therefore aims to summarize the current evidence and recommendations on when and how to evaluate vitamin D status in human health and disease. For the general population, most guidelines do not recommend universal screening but suggest a targeted approach in populations at risk. Also, some guidelines do not even recommend evaluating vitamin D status when vitamin D substitution is indicated anyway, such as in children or patients receiving anti-osteoporosis drugs. In those guidelines that recommend the screening of vitamin D status, serum 25(OH)D levels are universally proposed as the preferred screening tool. However, little attention is given to analytical considerations and almost no guidelines discuss the timing and frequency of screening. Finally, there is the known variability in diagnostic thresholds for defining vitamin D insufficiency and deficiency. Overall, the existing guidelines on the evaluation of vitamin D status differ broadly in screening strategy and screening implementation, and none of these guidelines discusses alternative screening modes, for instance, the vitamin metabolic ratio. Efforts to harmonize these different guidelines are needed to enhance their efficacy and cost-effectiveness.

Vitamin D Deficiency and the Presentation of Primary Hyperparathyroidism: A Mini Review

The clinical presentation of primary hyperparathyroidism (PHPT) has evolved over the years from a symptomatic disorder to a predominantly asymptomatic condition. Altered vitamin D metabolism seems to play a role in the presentation of PHPT and may exacerbate the severity of disease. The epidemiology of PHPT differs in the developing versus the developed world, where more severe phenotypes occur in regions where vitamin D deficiency is common. Although it has been validated that patients with PHPT should be vitamin D sufficient, the threshold to supplement in relation to the severity of PHPT and the degree of vitamin D deficiency remains controversial. This review will highlight some of the controversy regarding vitamin D deficiency and the different phenotypes of PHPT.

Levels of bioavailable, and free forms of 25(OH)D after supplementation with vitamin D3 in primary hyperparathyroidism

The 25 hydroxyvitamin D [25(OH)D] is the major metabolite for ascertaining vitamin D status, which circulates bound to a specific carrier (vitamin D-binding protein - VDBP). A portion that circulates unbound vary according to the VDBP genotype. This study evaluates the behavior of different forms of 25(OH)D, before and after supplementation with 14,000 IU of vitamin D3, weekly for 12 weeks, in individuals with primary hyperparathyroidism and controls. Fifty-six patients with active primary hyperparathyroidism (PHPT) and 64 paired controls (CTRL), not taking vitamin D3 for the last three months, were enrolled. The genetic isotypes of VDBP were determined to calculate bioavailable and free 25(OH)D. A p < 0.05 was considered significant. There were no statistical differences in free, bioavailable, and total 25(OH)D levels between PHPT and CTRL groups at baseline. The distribution of VDBP haplotypes 1s/1s, 1f/1f, 1s/1f, 2/2, 1s/2, and 1f/2 was similar between groups. After supplementation, all three forms of 25(OH)D proportionally increased within each group, although the percentage increment was lower in the PHPT group (p < 0.05). Total 25(OH)D is better correlated with PTH in the PHPT group than bioavailable and free 25(OH)D (r = -0.41; p < 0.05). The concentrations of total, free, and bioavailable 25(OH)D were similar in both PHPT and CTRL groups, and all forms increased proportionally after supplementation, although this increment percentage was higher in the CTRL group, with a subsequent reduction of PTH and AP. Total 25(OH)D correlated better with PTH than other forms, suggesting no advantages in measuring free or bioavailable 25(OH)D in these situations.

Concentrations of total, bioavailable, and free 25OHD in individuals with and without primary hyperparathyroidism and their correlations to DXA and trabecular bone score

Objective

This study aimed to investigate the association between 25OHD (total, bioavailable and free) with bone mass and microarchitecture among primary hyperparathyroidism (PHPT) patients and controls.

Subjects and methods

Sixty-four patients in the preoperative period of PHPT and 63 matched controls, who had not taken vitamin D in the last three months. To calculate the bioavailable and free 25OHD, the genetic variants of the vitamin D-binding protein (DBP) were determined. Bone mineral density (BMD) was determined by dual-energy X-ray absorptiometry (DXA). The distributions of total, bioavailable and free 25OHD and their correlation with TBS and DXA were evaluated.

Results

PHPT showed BMD and TBS values lower than CTRL in all locations (p < 0.05). There were no statistical differences in the levels of free, bioavailable and total 25OHD between the PHPT and CTRL groups [mean, min-max: 3.4 (1.4-8.6) vs. 3.1 (1.0- 9.8) pg/mL, 1.51 (0.43-3.58) vs. 1.41 (0.38-3.48) ng/mL, 22.6 (11.0-39.9) vs. 20.6 (8.9-35.3) ng/dL, respectively; (p > 0.05). The distribution of DBP haplotypes was similar between groups. DXA showed no correlation with any form of 25OHD in both groups. TBS presented a weak correlation with the total 25OHD in PHPT (r = 0.28; p = 0.02) and a moderate correlation with the total, free and bioavailable 25OHD in CTRL (r = 0.42; r = 0.42; r = 0.43; respectively, p < 0.01).

Conclusion

The concentrations of total, free and bioavailable 25OHD were similar in both the PHPT and control groups. 25OHD concentrations correlated positively with TBS and not with DXA, especially in controls, suggesting that this method may be more sensitive to assessing the consequences of vitamin D deficiency on bone quality in individuals without PHPT.

Vitamin D in dairy cows: metabolism, status and functions in the immune system

The function of vitamin D in calcium homoeostasis in dairy cows, such as in other vertebrates, is known for many years. In recent years, new and interesting, non-classical functions of vitamin D have been elucidated, including effects on the immune system. The major aim of this review is to provide an overview of effects of vitamin D or its metabolites on the immune system in dairy cows. The first part of the review provides an overview of vitamin D metabolism, with particular reference to the role of various proteins (25- and 1-hydroxylases, vitamin D binding protein, vitamin D receptor) in vitamin D signalling. The second part deals with the role of the concentration of 25-hydroxyvitamin D [25(OH)D] in plasma as an indicator of the vitamin D status in dairy cows, and its dependence on sunlight exposure and dietary vitamin D supplementation. In this part also the "free hormone hypothesis" is discussed, indicating that the concentration of free 25(OH)D might be a more valid indicator of the vitamin D status than the concentration of total 25(OH)D. The third part deals with classical and the non-classical functions of vitamin D. Among the non-classical functions which are based on an autocrine vitamin D signalling, particular reference is given to the effects of vitamin D and vitamin D metabolites on the immune system in bovine immune cells and in dairy cows. Recent findings provide some indication that vitamin D or its metabolite 25(OH)D could enhance the immune function in dairy cows and be useful for the prevention and therapy of mastitis. However, the number of studies reported so far in this respect is very limited. Thus, much more research is required to yield clear concepts for an optimised usage of vitamin D to improve the immune system and prevent infectious diseases in dairy cows.

[Vitamin D metabolite and calcium phosphorus metabolism in in patients with primary hyperparathyroidism on the background of bolus therapy with colecalciferol]

BACKGROUND

Vitamin D (25-hydroxyvitamin D [25(ОН)D]) deficiency (<20 ng/mL) and insufficiency (20-29 ng/mL) are common in primary hyperparathyroidism (PHPT), but data regarding the vitamin D metabolism in this population is limited.

AIM

The aim of this study is to estimate the vitamin D metabolites and their relationship with the main parameters of phosphorus-calcium metabolism in patients with PHPT at baseline and on the background of a single dose of cholecalciferol 150,000 IU.

MATERIALS AND METHODS

A single-center interventional, dynamic, prospective, comparative study has been carried out. The study included 54 participants, divided into two groups: the 1st group included 27 patients with confirmed PHPT, the 2nd control group (n = 27), matched on gender (p = 0.062). The study included 4 visits; the baseline laboratory examination and a bolus dose of cholecalciferol were performed at the visit 1, the subsequent visits included a dynamic laboratory examination.

RESULTS

Vitamin D deficiency (<20 ng/ml) was detected in 69% of patients with PHPT. In the PHPT group (before cholecalciferol therapy), there was a direct association of 1.25(OH)2 D3 with albumin-corrected and ionized calcium, as well as between the 25(OH)D3 /24.25(OH)2 D3 ratio with PTH and magnesium. After taking of cholecalciferol, the levels of 1.25(OH)2 D3 and 25(OH)D3 /24.25(OH)2 D3 were significantly increased, and the levels of 25(OH)D3 /1.25(OH)2 D3 were significantly declined at all visits among patients with PHPT. The common 25(OH)D level was comparable to the control group, however the levels of 1,25(OH)2 D3 in patients with PHPT were 55% higher at baseline, and after taking of cholecalciferol 150,000 IU. They remained increased by 3-7 days by an additional 23-36%, significantly higher than those in the control group: 44%, 74% and 65%, at visits 2, 3 and 4, respectively (p<0.05). The taking of 150,000 IU cholecalciferol in the PHPT group did not lead to a significant increase in hypercalcemia and hypercalciuria, which indicates the safety of this dose in patients with mild hypercalcemia (albumin corrected calcium <3 mmol/l). None of the study participants experienced any side effects.

CONCLUSION

The completely comprehensive assessment of vitamin D metabolites was carried out for the first time in patients with PHPT before and after using a bolus dose of cholecalciferol. The results confirmed the differences of vitamin D metabolism in chronic excessive secretion of PTH compared to control group, which is new data in the pathogenesis of the disease, and can be used to develop optimal regimens for cholecalciferol taking in this population.

Skeletal abnormalities in Hypoparathyroidism and in Primary Hyperparathyroidism

Both hypoparathyroidism (HypoPT), as well as its pathological counterpart, primary hyperparathyroidism (PHPT), can lead to skeletal abnormalities. Chronic deficiency of PTH in patients with HypoPT is associated with a profound reduction in bone remodeling, with consequent increases in bone density, and abnormalities in microarchitecture and bone strength. It is still not clear whether there is an increase in fracture risk in HypoPT. While standard therapy with calcium supplements and active vitamin D does not restore bone homeostasis, treatment of HypoPT with PTH appears to correct some of those abnormalities. In PHPT, the continuous exposure to high levels of PTH causes an increase in bone remodeling, in which bone resorption prevails. In the symptomatic form of PHPT, patients can present with fragility fractures, and/or the classical radiological features of osteitis fibrosa cystica. However, even in mild PHPT, catabolic skeletal actions of PTH are evident through reduced BMD, deterioration of bone microarchitecture and increased risk of fragility fractures. Successful parathyroidectomy improves skeletal abnormalities. Medical treatment, such as bisphosphonates and denosumab, can also increase bone density in patients with PHPT who do not undergo surgery. This article reviews skeletal involvement in HypoPT and in PHPT, as assessed by bone remodeling, DXA, trabecular bone score, and quantitative computed tomography, as well as data on bone strength and fracture risk. The effects of PTH replacement on the skeleton in subjects with HypoPT, and the outcome of parathyroidectomy in patients with PHPT, are also reviewed here.

Vitamin D: Dosing, levels, form, and route of administration: Does one approach fit all?

The 4th International Conference on Controversies in Vitamin D was held as a virtual meeting in September, 2020, gathering together leading international scientific and medical experts in vitamin D. Since vitamin D has a crucial role in skeletal and extra-skeletal systems, the aim of the Conference was to discuss improved management of vitamin D dosing, therapeutic levels and form or route of administration in the general population and in different clinical conditions. A tailored approach, based on the specific mechanisms underlying vitamin D deficiency in different diseases that were discussed, was recommended. Specifically, in comparison to healthy populations, higher levels of vitamin D and greater amounts of vitamin D were deemed necessary in osteoporosis, diabetes mellitus, obesity (particularly after bariatric surgery), and in those treated with glucocorticoids. Emerging and still open issues were related to target vitamin D levels and the role of vitamin D supplementation in COVID-19 since low vitamin D may predispose to SARS-CoV-2 infection and to worse COVID-19 outcomes. Finally, whereas oral daily cholecalciferol appears to be the preferred choice for vitamin D supplementation in the general population, and in most clinical conditions, active vitamin D analogs may be indicated in patients with hypoparathyroidism and severe kidney and liver insufficiency. Parenteral vitamin D administration could be helpful in malabsorption syndromes or in states of vitamin D resistance.Specific guidelines for desired levels of vitamin D should be tailored to the different conditions affecting vitamin D metabolism with the goal to define disease-specific normative values.

Recommendations on the measurement and the clinical use of vitamin D metabolites and vitamin D binding protein - A position paper from the IFCC Committee on bone metabolism

Vitamin D, an important hormone with a central role in calcium and phosphate homeostasis, is required for bone and muscle development as well as preservation of musculoskeletal function. The most abundant vitamin D metabolite is 25-hydroxyvitamin D [25(OH)D], which is currently considered the best marker to evaluate overall vitamin D status. 25(OH)D is therefore the most commonly measured metabolite in clinical practice. However, several other metabolites, although not broadly measured, are useful in certain clinical situations. Vitamin D and all its metabolites are circulating in blood bound to vitamin D binding protein, (VDBP). This highly polymorphic protein is not only the major transport protein which, along with albumin, binds over 99% of the circulating vitamin D metabolites, but also participates in the transport of the 25(OH)D into the cell via a megalin/cubilin complex. The accurate measurement of 25(OH)D has proved a difficult task. Although a reference method and standardization program are available for 25(OH)D, the other vitamin D metabolites still lack this. Interpretation of results, creation of clinical supplementation, and generation of therapeutic guidelines require not only accurate measurements of vitamin D metabolites, but also the accurate measurements of several other "molecules" related with bone metabolism. IFCC understood this priority and a committee has been established with the task to support and continue the standardization processes of vitamin D metabolites along with other bone-related biomarkers. In this review, we present the position of this IFCC Committee on Bone Metabolism on the latest developments concerning the measurement and standardization of vitamin D metabolites and its binding protein, as well as clinical indications for their measurement and interpretation of the results.

The Free Hormone Hypothesis: When, Why, and How to Measure the Free Hormone Levels to Assess Vitamin D, Thyroid, Sex Hormone, and Cortisol Status

The free hormone hypothesis postulates that only the nonbound fraction (the free fraction) of hormones that otherwise circulate in blood bound to their carrier proteins is able to enter cells and exert biologic effects. In this review, I will examine four hormone groups-vitamin D metabolites (especially 25OHD), thyroid hormones (especially thyroxine [T4]), sex steroids (especially testosterone), and glucocorticoids (especially cortisol)-that are bound to various degrees to their respective binding proteins-vitamin D-binding protein (DBP), thyroid-binding globulin (TBG), sex hormone-binding globulin (SHBG), and cortisol-binding globulin (CBG)-for which a strong case can be made that measurement of the free hormone level provides a better assessment of hormonal status than the measurement of total hormonal levels under conditions in which the binding proteins are affected in levels or affinities for the hormones to which they bind. I will discuss the rationale for this argument based on the free hormone hypothesis, discuss potential exceptions to the free hormone hypothesis, and review functions of the binding proteins that may be independent of their transport role. I will then review the complications involved with measuring the free hormone levels and the efforts to calculate those levels based on estimates of binding constants and levels of both total hormone and total binding protein. In this review, the major focus will be on DBP and free 25OHD, but the parallels and differences with the other binding proteins and hormones will be highlighted. Vitamin D and its metabolites, thyroid hormones, sex steroids, and glucocorticoids are transported in blood bound to serum proteins. The tightness of binding varies depending on the hormone and the binding protein such that the percent free varies from 0.03% for T4 and 25OHD to 4% for cortisol with testosterone at 2%. Although the major function of the primary carrier proteins (DBP, TBG, SHBG, and CBG) may be to transport their respective lipophilic hormones within the aqueous media that is plasma, these proteins may have other functions independent of their transport function. For most tissues, these hormones enter the cell as the free hormone presumably by diffusion (the free hormone hypothesis), although a few tissues such as the kidney and reproductive tissues express megalin/cubilin enabling by endocytosis protein-bound hormone to enter the cell. Measuring the free levels of these protein-bound hormones is likely to provide a better measure of the true hormone status than measuring the total levels in situations where the levels and/or affinities of the binding proteins are altered. Methods to measure free hormone levels are problematic as the free levels can be quite low, the methods require separation of bound and free that could disturb the steady state, and the means of separating bound and free are prone to error. Calculation of free levels using existing data for association constants between the hormone and its binding protein are likewise prone to error because of assumptions of linear binding models and invariant association constants, both of which are invalid. © 2020 The Author. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Low 25-Hydroxyvitamin D in Primary Hyperparathyroidism: Enhanced Conversion Into 1,25-Hydroxyvitamin D May Not Be "True" Deficiency

Vitamin D deficiency is reported in individuals with primary hyperparathyroidism (PHP). However, decreased 25OHD may be attributed to enhanced conversion into 1,25‐hydroxyvitamin D [1,25(OH)D]. To examine vitamin D metabolism in individuals with PHP, serum calcium, PTH, 25OHD, and 1,25(OH)D levels were determined in 210 adults: 102 with PHP, 40 with normal 25OHD, and 68 with vitamin D deficiency. Concentrations were redetermined in 37 individuals with PHP following vitamin D supplementation and 43 patients postsurgery. Comparisons were conducted by Student's t test and ANOVA. Correlations were assessed between PTH and 25OHD, 1,25(OH)D, and 1,25(OH)D/25OHD in individuals with PHP. Calcium, PTH, and 1,25(OH)D were higher (p < 0.001) in individuals with PHP (11.4 ± 0.4, 116 ± 21, 79 ± 6) than in individuals with normal 25OHD (9.6 ± 0.2, 49 ± 5, 57 ± 6) and vitamin D deficiency (9.3 ± 0.2, 62 ± 6, 32 ± 4). Compared with individuals with normal 25OHD (47 ± 5), 25OHD was lower (18 ± 3), but not different from subjects with vitamin D deficiency (15 ± 2). In individuals with PHP, vitamin D2 supplementation induced rises in 1,25(OH)D and calcium without lowering PTH, whereas postsurgery, calcium, PTH, 25OHD, and 1,25(OH)D normalized. Finally, in individuals with PHP, significant correlations (p < 0.01) were documented between PTH and calcium (r = 0.74), 25OHD (r = −0.43), 1,25(OH)D (r = 0.52), and 1,25(OH)D/25OHD (r = 0.46); and between 1,25(OH)D/25OHD and calcium (r = 0.47). Subnormal 25OHD in most individuals with PHP may be attributed to enhanced conversion to 1,25(OH)D—not “true” vitamin D deficiency—although in some patients, both PHP and vitamin D deficiency coexisted. Moreover, vitamin D supplementation exaggerated hypercalcemia in individuals with PHP. © 2020 The Author. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Unbound Vitamin D Concentrations Are Not Decreased in Critically Ill Patients

OBJECTIVE

Free concentrations of highly protein bound hormones, such as cortisol and thyroxine, are unchanged in critical illness despite substantial decreases in total concentration. Total 25-hydroxyvitamin D (25(OH)D) concentration is decreased in critical illness, but the free concentration of 25(OH)D has had less attention. The aim of this study was to compare total and calculated free 25(OH)D concentrations in critically ill patients with healthy controls.

DESIGN

Case control study.

METHODS

38 patients with critical illness were compared with 68 healthy controls. 25(OH)D was measured by liquid chromatography tandem mass spectrometry (LCMS/MS) and vitamin D binding protein (VDBP) by direct sandwich enzyme-linked immunosorbent assay (ELISA). Total and calculated free 25(OH)D concentrations were compared using unpaired T-tests.

RESULTS

Total 25(OH)D concentrations were significantly lower in critically ill patients than controls (37 (95% CI 31 - 43) vs 57 (53 - 60) nmol/L). Calculated free concentrations of 25(OH)D were not lower in critically ill patients than healthy controls (26 (22 - 29) vs 19 (18 - 20) pmol/L).

CONCLUSIONS

Calculated free 25(OH)D concentrations are not decreased in critical illness. Measuring total 25(OH)D concentrations in patients with critical illness potentially underestimates vitamin D and overestimates the number of patients who are deficient in vitamin D. This article is protected by copyright. All rights reserved.

Vitamin D deficiency is associated with reduced mobility after hip fracture surgery: a prospective study

BACKGROUND

Hip fractures are associated with a high rate of morbidity and mortality, and successful ambulation after surgery is an important outcome in this patient population.

OBJECTIVE

This study aims to determine whether 25-hydroxyvitamin D [25(OH)D] concentration or the Geriatric Nutritional Risk Index (GNRI) is associated with mortality or rates of walking in a patient cohort after hip fracture surgery.

METHODS

Patients undergoing hip fracture repair from a multisite study in North America were included. Mortality and mobility were assessed at 30 and 60 d after surgery. Serum albumin, 25(OH)D, and intact parathyroid hormone were measured. Patients were characterized according to 25(OH)D <12 ng/mL, 12 to <20 ng/mL, 20 to <30 ng/mL, or ≥30 ng/mL. GNRI was categorized into major/moderate nutritional risk (<92), some risk (92 to <98), or in good nutritional status (≥98).

RESULTS

Of the 290 patients [aged 82 ± 7 y, BMI (kg/m2): 25 ± 5], 73% were women. Compared with patients with <12 ng/mL, those with higher 25(OH)D concentrations had higher rates of walking at 30 d (P = 0.031): 12 to <20 ng/mL (adjusted OR: 2.61; 95% CI: 1.13, 5.99); 20 to <30 ng/mL (3.48; 1.53, 7.95); ≥30 ng/mL (2.84; 1.12, 7.20). In addition, there was also greater mobility at 60 d (P = 0.028) in patients with higher 25(OH)D compared with the reference group (<12 ng/mL). Poor nutritional status (GNRI <92) showed an overall trend to reduce mobility (unadjusted P = 0.044 and adjusted P = 0.056) at 30 but not at 60 d. There was no association of vitamin D or GNRI with mortality at either time.

CONCLUSIONS

Vitamin D deficiency (<12 ng/mL) is associated with reduced ambulation after hip fracture surgery, whereas GNRI also contributes to immobility but is a less reliable predictor. Mechanisms that can explain why vitamin D deficiency is associated with mobility should be addressed in future studies.

The measurement of vitamin D metabolites part II-the measurement of the various vitamin D metabolites

Today, the possibility exists to measure a number of different vitamin D metabolites with accurate and precise methods. The most abundant vitamin D metabolite, 25(OH)D, is considered the best marker for estimating vitamin D status and is therefore the most commonly measured in clinical practice. There is no consensus on the added value of measuring other metabolites beyond 25-hydroxyvitamin D, although, in some special clinical scenarios and complicated cases, these metabolites may provide just the information needed for an accurate diagnosis. The problem this review addresses is which metabolite to measure and when and how to measure it.

New aspects of vitamin D metabolism and action - addressing the skin as source and target

Vitamin D has a key role in stimulating calcium absorption from the gut and promoting skeletal health, as well as many other important physiological functions. Vitamin D is produced in the skin. It is subsequently metabolized to its hormonally active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D), by the 1-hydroxylase and catabolized by the 24-hydroxylase. In this Review, we pay special attention to the effect of mutations in these enzymes and their clinical manifestations. We then discuss the role of vitamin D binding protein in transporting vitamin D and its metabolites from their source to their targets, the free hormone hypothesis for cell entry and HSP70 for intracellular transport. This is followed by discussion of the vitamin D receptor (VDR) that mediates the cellular actions of 1,25(OH)₂D. Cell-specific recruitment of co-regulatory complexes by liganded VDR leads to changes in gene expression that result in distinct physiological actions by 1,25(OH)₂D, which are disrupted by mutations in the VDR. We then discuss the epidermis and hair follicle, to provide a non-skeletal example of a tissue that expresses VDR that not only makes vitamin D but also can metabolize it to its hormonally active form. This enables vitamin D to regulate epidermal differentiation and hair follicle cycling and, in so doing, to promote barrier function, wound healing and hair growth, while limiting cancer development.

Parathyroid gland weight is associated with high density lipoprotein levels in patients with primary hyperparathyroidism

Objective

To investigate the relationship between parathyroid gland weight and high-density lipoprotein (HDL) levels in patients with primary hyperparathyroidism (PHPT).

Methods

In this retrospective case control study, we reviewed 329 PHPT patients aged from 20 to 85 years who had a parathyroidectomy at Robert Wood Johnson University Hospital. The patients were divided into 5 quintiles according to their parathyroid gland weight: 68 patients had a parathyroid gland weight <0.3 g, 66 patients had a gland weight 0.3–0.45 g, 67 patients had a gland weight 0.45–0.7 g, 63 patients had a gland weight 0.7–1.25 g, and 65 patients had a gland weight ≥1.25 g.

Results

Body Mass Index (BMI) trended to be higher across the quintiles of parathyroid gland weight (P = 0.003). Serum calcium and PTH levels were significantly increased across parathyroid gland quintiles (p < 0.0001). HDL levels tended to be lower across the increasing quintiles of parathyroid gland weight (P = 0.01). There was a negative relationship between log parathyroid gland weight and HDL in patients with PHPT in a simple linear regression (r = −0.160, P = 0.003). The negative association remained significant after adjustment for age and BMI (r = −0.114, P = 0.039). Furthermore, parathyroid gland weight was significantly associated with levels of triglyceride (r = 0.126, P = 0.02), but this relationship lost its significance after adjustment for age and BMI (r = 0.082, P ˃ 0.05).

Conclusions

PHPT patients with heavier parathyroid glands tended to have higher BMI and lower HDL levels.

Total, Bioavailable, and Free Vitamin D Levels and Their Prognostic Value in Pulmonary Arterial Hypertension

Introduction: Epidemiological studies suggest a relationship between vitamin D deficiency and cardiovascular and respiratory diseases. However, whether total, bioavailable, and/or free vitamin D levels have a prognostic role in pulmonary arterial hypertension (PAH) is unknown. We aimed to determine total, bioavailable, and free 25-hydroxy-vitamin D (25(OH)vitD) plasma levels and their prognostic value in PAH patients. Methods: In total, 67 samples of plasma from Spanish patients with idiopathic, heritable, or drug-induced PAH were obtained from the Spanish PH Biobank and compared to a cohort of 100 healthy subjects. Clinical parameters were obtained from the Spanish Registry of PAH (REHAP). Results: Seventy percent of PAH patients had severe vitamin D deficiency (total 25(OH)vitD < 10 ng/mL) and secondary hyperparathyroidism. PAH patients with total 25(OH)vitD plasma above the median of this cohort (7.17 ng/mL) had better functional class and higher 6-min walking distance and TAPSE (tricuspid annular plane systolic excursion). The main outcome measure of survival was significantly increased in these patients (age-adjusted hazard ratio: 5.40 (95% confidence interval: 2.88 to 10.12)). Vitamin D-binding protein (DBP) and albumin plasma levels were downregulated in PAH. Bioavailable 25(OH)vitD was decreased in PAH patients compared to the control cohort. Lower levels of bioavailable 25(OH)vitD (<0.91 ng/mL) were associated with more advanced functional class, lower exercise capacity, and higher risk of mortality. Free 25(OH)vitD did not change in PAH; however, lower free 25(OH)vitD (<1.53 pg/mL) values were also associated with high risk of mortality. Conclusions: Vitamin D deficiency is highly prevalent in PAH, and low levels of total 25(OH)vitD were associated with poor prognosis.

Comparison of free and total 25-hydroxyvitamin D in normal human pregnancy

Vitamin D status correct monitoring during pregnancy is critically important for both maternal and fetal health. 25-Hydroxyvitamin D (25(OH)D) - a prohormone of a biologically active 1,25-dihydroxyvitamin D (1,25(OH)₂D), despite the lack of biological activity, during the past decades has been routinely used as a main biomarker characterizing vitamin D status. About 85% of 25(OH)D in the bloodstream is bound to its specific carrier - vitamin D-binding protein (DBP), the remaining 15% are loosely bound to albumin, and only less than 0.1% are free in the circulation ("free 25(OH)D"). Total 25(OH)D is the sum of DBP-bound, albumin-bound and free 25(OH)D. According to a "free hormone hypothesis", only free 25(OH)D is able to induce a biological effect. Normal pregnancy is characterized by elevated serum DBP levels, and due to this fact the diagnostic strength of serum total 25(OH)D has been questioned. Free 25(OH)D might be a better characteristic of vitamin D status in this settings. We aimed to compare the diagnostic strength of a routine total 25(OH)D with directly measured free 25(OH)D in normal pregnancy by comparing the association strength between free and total 25(OH)D with biomarkers of bone health (PTH, calcium, bone-specific alkaline phosphatase (BSAP)), lipid metabolism (adiponectin, LDL, HDL), kidney function (urea), endocrine parameters (T4, T3, TSH), and group B water-soluble vitamins. The study was conducted in 368 healthy white pregnant women - residents of north-east Germany. Free 25(OH)D showed an overall better associations with gestational age, markers of bone metabolism (calcium (rho = 0.141, p = 0.007 with free 25(OH)D; rho = 0.060, p = 0.251 with total 25(OH)D) and BSAP (rho = -0.203, p < 0.001 with free 25(OH)D; rho = -0.108, p = 0.038 with total 25(OH)D), lipid metabolism parameters (adiponectin (rho = 0.142, p = 0.008 with free 25(OH)D; rho = 0.054, p = 0.307 with total 25(OH)D), LDL cholesterol (rho = -0.191, p < 0.001 with free 25(OH)D; rho = 0.033, p = 0.539 with total 25(OH)D)) and a kidney function marker (urea (rho = 0.114, p = 0.032 with free 25(OH)D; rho = 0.008, p = 0.887 with total 25(OH)D)) than total 25(OH)D. In conclusion, the current study revealed that free 25(OH)D is a more precise determinant of the vitamin D status during normal human pregnancy than total 25(OH)D. In the settings of normal pregnancy, free 25(OH)D revealed better associations with markers of bone metabolism (calcium, BSAP), lipid metabolism (adiponectin, LDL cholesterol, LDL/HDL ratio) and kidney function (urea) than total 25(OH)D.

Vitamin D Binding Protein, Total and Free Vitamin D Levels in Different Physiological and Pathophysiological Conditions

This review focuses on the biologic importance of the vitamin D binding protein (DBP) with emphasis on its regulation of total and free vitamin D metabolite levels in various clinical conditions. Nearly all DBP is produced in the liver, where its regulation is influenced by estrogen, glucocorticoids and inflammatory cytokines but not by vitamin D itself. DBP is the most polymorphic protein known, and different DBP alleles can have substantial impact on its biologic functions. The three most common alleles-Gc1f, Gc1s, Gc2-differ in their affinity with the vitamin D metabolites and have been variably associated with a number of clinical conditions. Although DBP has a number of biologic functions independent of vitamin D, its major biologic function is that of regulating circulating free and total levels of vitamin D metabolites. 25 hydroxyvitamin D (25(OH)D) is the best studied form of vitamin D as it provides the best measure of vitamin D status. In a normal non-pregnant individual, approximately 0.03% of 25(OH)D is free; 85% is bound to DBP, 15% is bound to albumin. The free hormone hypothesis postulates that only free 25(OH)D can enter cells. This hypothesis is supported by the observation that mice lacking DBP, and therefore with essentially undetectable 25(OH)D levels, do not show signs of vitamin D deficiency unless put on a vitamin D deficient diet. Similar observations have recently been described in a family with a DBP mutation. This hypothesis also applies to other protein bound lipophilic hormones including glucocorticoids, sex steroids, and thyroid hormone. However, tissues expressing the megalin/cubilin complex, such as the kidney, have the capability of taking up 25(OH)D still bound to DBP, but most tissues rely on the free level. Attempts to calculate the free level using affinity constants generated in a normal individual along with measurement of DBP and total 25(OH)D have not accurately reflected directly measured free levels in a number of clinical conditions. In this review, we examine the impact of different clinical conditions as well as different DBP alleles on the relationship between total and free 25(OH)D, using only data in which the free 25(OH)D level was directly measured. The major conclusion is that a number of clinical conditions alter this relationship, raising the question whether measuring just total 25(OH)D might be misleading regarding the assessment of vitamin D status, and such assessment might be improved by measuring free 25(OH)D instead of or in addition to total 25(OH)D.

Vitamin D Binding Protein and the Biological Activity of Vitamin D

Vitamin D has a long-established role in bone health. In the last two decades, there has been a dramatic resurgence in research interest in vitamin D due to studies that have shown its possible benefits for non-skeletal health. Underpinning the renewed interest in vitamin D was the identification of the vital role of intracrine or localized, tissue-specific, conversion of inactive pro-hormone 25-hydroxyvitamin D [25(OH)D] to active 1,25-dihydroxyvitamin D [1,25(OH)₂D]. This intracrine mechanism is the likely driving force behind vitamin D action resulting in positive effects on human health. To fully capture the effect of this localized, tissue-specific conversion to 1,25(OH)₂D, adequate 25(OH)D would be required. As such, low serum concentrations of 25(OH)D would compromise intracrine generation of 1,25(OH)₂D within target tissues. Consistent with this is the observation that all adverse human health consequences of vitamin D deficiency are associated with a low serum 25(OH)D level and not with low 1,25(OH)₂D concentrations. Thus, clinical investigators have sought to define what concentration of serum 25(OH)D constitutes adequate vitamin D status. However, since 25(OH)D is transported in serum bound primarily to vitamin D binding protein (DBP) and secondarily to albumin, is the total 25(OH)D (bound plus free) or the unbound free 25(OH)D the crucial determinant of the non-classical actions of vitamin D? While DBP-bound-25(OH)D is important for renal handling of 25(OH)D and endocrine synthesis of 1,25(OH)₂D, how does DBP impact extra-renal synthesis of 1,25(OH)₂D and subsequent 1,25(OH)₂D actions? Are their pathophysiological contexts where total 25(OH)D and free 25(OH)D would diverge in value as a marker of vitamin D status? This review aims to introduce and discuss the concept of free 25(OH)D, the molecular biology and biochemistry of vitamin D and DBP that provides the context for free 25(OH)D, and surveys in vitro, animal, and human studies taking free 25(OH)D into consideration.

25-Hydroxyvitamin D and Vitamin D Binding Protein Levels in Patients With Primary Hyperparathyroidism Before and After Parathyroidectomy

Objective: To evaluate vitamin D binding protein and free 25-hydroxyvitamin D [25(OH)D] levels in healthy controls compared to primary hyperparathyroidism (PHPT) patients, and to examine PHPT before and after surgery. Methods: Seventy-five PHPT patients and 75 healthy age, gender, and body mass index (BMI) -matched control subjects were examined. In addition, 25 PHPT patients underwent parathyroidectomy and had a 3-month follow up visit. Levels of total and free 25(OH)D, DBP, and intact parathyroid hormone (iPTH) were determined before and 3 months after surgery. Results: There was no significant difference in age and BMI between PHPT patients and controls. Levels of 25(OH)D and DBP were lower in PHPT patients compared to controls (p < 0.01). There was no significant difference in calculated free and bioavailable 25(OH)D levels between PHPT patients and controls. Calcium and iPTH levels decreased to normal but DBP and DBP-bound-25(OH)D increased (P < 0.001) after parathyroidectomy. Levels of DBP were inversely correlated with iPTH (r = -0.406, P < 0.001) and calcium levels (r = -0.423, P < 0.001). Conclusion: Serum DBP levels were lower in patients with PHPT and parathyroidectomy restored DBP levels. We suggest that lower DBP levels is one of contributing mechanisms of low total 25(OH)D in PTHP patients and the total 25(OH)D levels might not reflect true vitamin D status in PHPT patients.

Primary Hyperparathyroidism

Background

Primary hyperparathyroidism (PHPT), the most common cause of hypercalcemia, is most often identified in postmenopausal women. The clinical presentation of PHPT has evolved over the past 40 years to include three distinct clinical phenotypes, each of which has been studied in detail and has led to evolving concepts about target organ involvement, natural history, and management.

Methods

In the present review, I provide an evidence-based summary of this disorder as it has been studied worldwide, citing key concepts and data that have helped to shape our concepts about this disease.

Results

PHPT is now recognized to include three clinical phenotypes: overt target organ involvement, mild asymptomatic hypercalcemia, and high PTH levels with persistently normal albumin-corrected and ionized serum calcium values. The factors that determine which of these clinical presentations is more likely to predominate in a given country include the extent to which biochemical screening is used, vitamin D deficiency is present, and whether parathyroid hormone levels are routinely measured in the evaluation of low bone density or frank osteoporosis. Guidelines for parathyroidectomy apply to all three clinical forms of the disease. If surgical guidelines are not met, parathyroidectomy can also be an appropriate option if no medical contraindications are present. If either the serum calcium or bone mineral density is of concern and surgery is not an option, pharmacological approaches are available and effective.

Conclusions

Advances in our knowledge of PHPT have guided new concepts in diagnosis and management.

Comparison of Initial Clinical Presentations between Primary Hyperparathyroidism Patients from New Brunswick and Changsha

PURPOSE

To compare the initial clinical features, laboratory values, and bone mineral density among patients with primary hyperparathyroidism (PHPT) in Changsha (China) and New Brunswick (USA).

METHODS

In this retrospective study, we reviewed 169 PHPT patients who presented at Robert Wood Johnson University Hospital and 133 PHPT patients who presented at the Second Xiangya Hospital of Central South University in the same time period. The following characteristics were compared between the groups: age, gender, BMI, serum calcium, alkaline phosphatase (AKP), albumin, intact PTH (iPTH), 25-hydroxyvitamin D (25 (OH) D), fasting blood glucose levels, and bone mineral density (BMD). All these parameters were also compared according to gender and menopausal status. iPTH associations were also assessed along with several other parameters.

RESULTS

PHPT patients from Changsha had higher serum calcium, iPTH, and AKP levels but lower 25 (OH) D levels than the patients from New Brunswick (p < 0.05). Patients in Changsha had lower T-scores and Z-scores in both the lumbar spine and hip regions than those in New Brunswick (p < 0.05). Patients in New Brunswick had lower percentages of parathyroid adenoma and kidney stones. Serum iPTH level was positively correlated with serum calcium and serum AKP levels in both Changsha and New Brunswick (p < 0.05).

CONCLUSIONS

There are distinct biochemical and clinical differences between patients with PHPT in China and the United States. Our study revealed that Asian PHPT patients from Changsha presented more severe PHPT profiles, lower bone mineral density, and higher incidence of renal stones.

VITAMIN D BINDING PROTEIN AND 25-HYDROXYVITAMIN D LEVELS: EMERGING CLINICAL APPLICATIONS

The precursor of the active form of vitamin D, 25-hydroxyvitamin D (25(OH)D), is recognized as the optimal indicator of vitamin D status. Vitamin D3 undergoes conversion through a multitude of enzymatic reactions described within the paper, and vitamin D levels are dependent on many factors including the vitamin D binding protein (DBP). The free hormone hypothesis postulates that protein-bound hormones are not biologically available and that unbound hormones are biologically active. The majority of circulating 25(OH)D and 1,25-dihydroxyvitamin D is tightly bound to DBP and albumin, with less than 1% circulating in an unbound form. As a result, factors affecting DBP alter the interpretation of 25(OH)D levels. The aim of this review is to assess the current methodology used to measure total and free 25(OH)D, and DBP. Additionally, we analyze the effects of other endocrine hormones and disease processes on DBP levels and subsequently, the interpretation of 25(OH)D levels.

ABBREVIATIONS

CF = cystic fibrosis DBP = vitamin D binding protein ELISA = enzyme-linked immunosorbent assay ESLD = end-stage liver disease HC = hormone contraceptives iPTH = intact parathyroid hormone LC-MS = liquid chromatography-mass spectrometry MS = multiple sclerosis 25(OH)D = 25-hydroxyvitamin D PHPT = primary hyperparathyroidism RIA = radioimmunoassay.