The importance of parathormone in determining the deficiency of vitamin D

Introduction

Vitamin D has both skeletal and extra skeletal functions. It affects the intestinal calcium absorption and provides the skeleton formation and continuity [1]. Vitamin D deficiency is frequently seen in the adults [2] and causes osteomalacia and osteoporotic fractures [3]. Other than the effects on the bone Vitamin D displays immunomodulatory, antiproliferative and anti-inflammatory features [4]. Although there is no determined an ideal grade for serum 25-OH vitamin D, it is accepted that level above 30 ng/mL is adequate, 20–30 ng/mL is insufficiency and level under 20 ng/mL is deficiency [5], [6]. If there are no primer parathyroid pathologies vitamin D deficiency causes to seconder hyperparathyroidism, so high value of parathormone (PTH) can be used a representative marker for determining vitamin D lack [7]. Sahota et al. [8] indicated that vitamin D deficiency is related both 25-OH vitamin D threshold value and the 25-OH vitamin D levels which increases PTH.

In our research we intended to evaluate the relationship between PTH with vitamin D and other biochemical parameters [calcium (Ca), alkaline phosphatase (ALP), phosphor (P), creatinine (Cr)] and also find a cut-off value for 25-OH vitamin D. We purposed to show the important of PTH to determine vitamin D deficiency.

Material and methods

Our study is a cross-sectional, retrospective study. The data was obtained from the biochemistry laboratory registry system of the medical faculty hospital of university. Patients who had measurement of serum PTH level between 2010 and 2014 years were taken into consideration. Patients who had no contemporaneous measure of 25-OH vitamin D and PTH were not incorporated. Accordingly hospital reference range patients with serum albumin value <4 mg/dL or a creatinine value >1.3 mg/dL which can make a sign a prior or the present disease were not included. Data of patients from oncology, organ transplant unit, nephrology, endocrinology and pediatrics department were excluded.

The patient’s age, gender and month the blood examination was performed were recorded from the database system. Serum 25-OH vitamin D, P, ALP, albumin, Ca, Cr results were recorded. 25-OH vitamin D levels were measured by chemoluminescence immunoassay method on Roche Diagnostics Cobas 6000 instrument, as ng/mL. PTH levels were measured by chemoluminescence immunoassay method on Roche Diagnostics Cobas 6000 instrument as pg/mL. Serum PTH reference interval was accepted from 0 to <65 pg/mL as normal and PTH ≥65 pg/mL as hyperparathyroidism according to our hospital reference range. Patients were grouped in accordance with different features such as age, 25-OH vitamin D status and calcium levels. Based on ages the patients were separated to 7 decades which were into 3^rd to 9^th decades and above. Based on Turkey endocrinology and metabolism, osteoporosis and metabolic bone disease association guide the 25-OH vitamin D levels were separated into 4 groups; as <10 ng/mL severe deficiency, 10–20 ng/mL deficiency, 20–30 ng/mL insufficiency, >30 pg/mL normal. According to serum calcium levels 5 groups were formed as <8 mg/dL, 8.01–9 mg/dL, 9.01–10 mg/dL, 10.01–11 mg/dL, >11 mg/dL. Serum PTH levels were evaluated in relation to age, gender, month, 25-OH vitamin D and other biochemical parameters.

Results

A total of 11,753 (2352 males, 9401 females) subjects’ data was compassed. The median age of the subjects was 53 years (minimum 20, maximum 95) and the mean PTH level was 49.33±22.39 pg/mL. Demographic and biochemical characteristics of the patients are shown in Table 1. PTH examinations were done more between the first and 6^th months of the year. Demographic and biochemical characteristics of patients with and without hyperparathyroidism are submitted in Table 2. The 25-OH vitamin D level was ≤30 ng/mL in 9094 (77.4%) patients, 21.3% (2495) patients had severe deficiency, 31% (3649) patients had deficiency and 25-OH vitamin D was insufficient level in 25.1% (2950) patients. The 25-OH vitamin D level was >30 ng/mL in 2659 (22.6%) patients (Table 3). The frequencies of the 25-OH vitamin D status in male and female patients are shown in Table 3. 25-OH vitamin D levels were significantly lower in female patients (p<0.05).

There was statistically significant a negative correlation between PTH and 25- OH vitamin D levels (p=0.000, r=−0.255). The correlation was weak. We found 25-OH vitamin D level which increased the PTH value above the reference interval was 18.5±0.38 ng/mL (99% CI) (Figure 1). According to the hospital standard laboratory reference interval, 18.7% of patients had hyperparathyroidism. In 87.7% (n=1929) of patients with hyperparathyroidism 25-OH vitamin D was ≤30 ng/mL (severe deficient in 37.9%, deficient in 32.7%, insufficient in 17.1%). The PTH level was significantly different between women and according to 25-OH vitamin D groups (p<0.05). There was hyperparathyroidism in 271 (10.2%) patients whose 25-OH vitamin D was >30 ng/mL. In 7165 (75%) of patients who have normal PTH levels vitamin D were deficient. PTH levels relative to 25-OH vitamin D groups are shown in Table 4. The frequency of hyperparathyroidism in 25-OH vitamin D groups are denoted in Table 5. We found a cut-off point of 18.5 with a sensitivity of 73%, a specificity of 56%, a positive predictive value (PPV) of 25%, and a negative predictive value (NPV) of 88% for 25-OH vitamin D (Table 6).

Figure 1:

The 25-OH vitamin D level, which increases the PTH value above the reference interval was18.5±0.38 ng/mL (99% CI).

The mean 25-OH vitamin D of patients in the third decade was 20.17±12.43 ng/mL, and in patients with 9^th and over decade was 23.05±16.12 ng/mL. We found a positive correlation between 25-OH vitamin D levels and decades (r=0.93, p<0.05). The distribution frequency of the 25-OH vitamin D level status according to decade is denoted in Figure 2. The PTH levels also increased with decades (Figure 3). The lowest 25-OH vitamin D values were detected in January and February, the highest level were between July and September, there was significant difference (p<0.05). The lowest PTH values were in August and September, the highest were in March and April. According to months, 25-OH vitamin D and PTH levels are denoted in Table 7. The mean P level was lower in patients with 25-OH vitamin D level ≤30 ng/mL (3.58±0.57 mg/dL) than with 25-OH vitamin D level >30 ng/mL (3.60±0.58 mg/dL) but difference was not significant. Patients with non-hyperparathyroidism have higher P level (p<0.05). The ALP value was higher in patients with 25-OH vitamin D level ≤30 ng/mL (86.001±39.59 U/mL) compared to patients with >30 ng/mL (81.158±36.41 U/mL). The difference was statistically significant (p<0.05). The ALP value was higher in hyperparathyroidism group (p<0.05). The mean calcium was higher in patients with normal level 25-OH vitamin D (p<0.05).

Figure 2:

Frequency of 25 OH vitamin D status in decades.

Figure 3:

The mean PTH levels according to decades.

The calcium group with 9.01−10 mg/dL and group with 10.01−11 mg/dL showed a significant negative (p<0.05) and positive correlation (p<0.05) with PTH, respectively. Although a positive correlations were observed between calcium and PTH in <8 mg/dL, 8.01−9.00 mg/dL and >11 mg/dL calcium groups the correlations were not significant.

Discussion

In our study, we studied relation between PTH and 25-OH vitamin D and other biochemical parameters and determined 25-OH vitamin D level that would keep PTH level below hospital reference range. Secondary hyperparathyroidism is defined as a answer to a low calcium level concerned with hypovitaminosis D [9]. Therefore, the description of secondary hyperparathyroidism is attached to measure of the amount of 25-OH vitamin D deficiency or deficiency associated with PTH elevation [10]. Until today 25-OH vitamin D levels are widely investigated although there are few studies evaluating the relationship between 25-OH vitamin D and serum PTH. Souberbielle et al. [11] found that 80% of the participants in the study conducted in 892 healthy adults participants aged from 18 to 89 had 25-OH vitamin D levels <30 ng/mL (in 6.3% <10 ng/mL, in 9.9% <12 ng/mL, 34.6% <30 ng/mL). Vitamin D related several studies were conducted in Turkey [12], [13]. Uçar et al. [12] took 20 ng/mL as a cut-off value for 25-OH vitamin D in a research involving 513 patients and found 51.8% deficiency and 20.7% insufficiency 25-OH vitamin D and associated with inadequate exposure to sunlight and dietary factors. In our study majority of participant (77.4%) had 30 ng/mL or less 25-OH vitamin D levels and particularly there was a deficiency or serious deficiency in a significant part of them. Relationship between PTH and increase age was investigated. Age related increasing in PTH level has been shown in men [14] and women [15], [16]. In this research serum PTH levels increased with ageing. This was compatible with the literature. Patients in 9^th decade and above had the highest PTH level. Farrel et al. [17] tried to find age-related reference interval for PTH and found 63% increase in the upper and lower reference limits between the youngest and the oldest participators. This study implicated that older age was related with higher PTH in patients who had optimal 25-OH vitamin D.

Parathormone is shown to be increased with increase creatinine [18]. Orwoll et al. [14] indicated that renal function markedly reduces and metabolism of vitamin D alters with increase age in normal men. In our study we excluded the impression of serum creatinine on serum PTH levels because we excluded the patients with elevated creatinine. Therefore we thought that the increase in parathormone with age may be due to the change in vitamin D metabolism. Studies evaluating parathormone’s relationship with gender showed different results. In contrast to some studies showing that PTH levels were not related to gender [18], [19]. PTH levels were significantly higher in female patients compared to male patients in our study (p<0.05). Serdar et al. [20] demonstrated that PTH values significantly higher in female patients. Farrell et al. [17] assessed gender-related relationships of PTH excluding cases with 25-OH vitamin D <50 nmol/L and found higher levels PTH in female patients, but did not find any statistical difference. The inverse correlation between 25-OH vitamin D and PTH was shown in researches. Kim et al. [21] demonstrated that 25-OH vitamin D had significant a negative correlation with PTH in women and men (r=−0.249, r=−0.228, p<0.001). Wang et al. [22] found that lower 25-OH D was related higher PTH (r=−0.126, p<0.001). But PTH elevation may not be observed in all patients who have 25-OH vitamin D deficiency. Sahota et al. [8] found 39% hypovitaminosis D and showed secondary hyperparathyroidism in only 33% of them in a study which included postmenopausal women with founded osteoporosis. The subjects who had low 25-OH vitamin D but not had elevated PTH level were called patients with functional hypoparathyroidism. We detected statistical significant a negative correlation between 25-OH vitamin D and serum PTH levels (p=0.000, r=−0.255) and power of correlation was like other researches. As the severity of vitamin D deficiency increased, the frequency of hyperparathyroidism was increasing in our study. The frequency of hyperparathyroidism was the most common in severe vitamin D deficiency group. The highest PTH levels was found in the group of severe deficiency. The effect of gender on 25-OH vitamin D levels is contentious. Although there are studies proving that 25-OH vitamin D is lower in women [23], [24], a meta-analysis reported that women had a higher vitamin D level than men [25]. In our study the women participants had lower 25-OH vitamin D level compared with men.

Vitamin D deficiency is widespread in the senior people due to numerous factors [23]. In a study 25-OH vitamin concentrations were investigated in 824 elderly people from 11 European countries in winter and showed that vitamin D levels were <30 nmol/L in 36% of men and 47% of women. The lowest mean 25-OH vitamin D concentration was seen in southern countries and emphasized that elderly people were at risk of low vitamin D irrespective of geographical area [24]. Martins et al. [26] showed the higher median values were observed for individuals between 0 and 20 years old and the lowest median values were observed in 70 years or older. In 30.3% of older than 70 individuals had lower than 20 ng/mL 25-OH vitamin D. Lu et al. [27] demonstrated that proportion of vitamin D deficiency was 69.2% in study group which contained subjects aged 50–70 years. Unlike studies 25-OH vitamin D levels increased with age in our study. The frequency of normal 25-OH vitamin D status increased as the 3^rd decade went to 9^th and above decades. This result was thought to be due to the fact that older patients could be taking vitamin D supplements and adequate exposure to sunlight.

Vitamin D levels have seasonal changeability [28]. Effect of seasons investigated in different studies. Piirainen et al. [29] showed that 25-OH vitamin D levels were 42% higher in summer compared to winter in the research which was done on 69 healthy subjects living at latitudes with extremely variable seasonal exposure to sunlight. Hekimsoy et al. [30] investigated 25-OH vitamin D values in Aegean region of Turkey in February and stated that 25-OH vitamin D was under 20 ng/mL in 74.9% and 20–29.99 ng/mL in 13.8% of the participants. This study showed that only 11.3% of participants had sufficient status. In our study, 25-OH vitamin D levels measured during in summer were significantly higher than in winter period this result was similar to other studies. Serum PTH in relation to seasons was appraised in a small number of studies. Krall et al. [31] found the lowest levels of PTH in August–October and the highest levels in March–May. Melin et al. [32] showed seasonal associations with intact PTH levels in subjects with 3 or more hours of outdoor activity per week. We found the lowest levels of PTH in August–October and highest the levels in March and April. This finding suggests effect of seasons.

Vitamin D level that causes parathormone increases are defined as vitamin D deficiency [33]. To definition of insufficiency, the 25-OH vitamin D and PTH values should be take into account together [8]. Several studies pointed out the importance of PTH in determining the deficiency of vitamin D and determined a new threshold 25-OH vitamin D value. Sai et al. [34] evaluated a threshold value of serum 25-OH vitamin D by using PTH level. The study showed that serum PTH continuously decreased as 25-OH vitamin D increased from 6 to 60 ng/mL. Vitamin D level under than 20 ng/ml was defined insufficiency. Bouillon et al. [35] underlined important of the threshold of serum 25-OH vitamin D below which serum PTH starts to increase. Sahin et al. [36] committed point of serum 25-OH vitamin D level suppressed of PTH was at 30 ng/mL. Arvizu et al. [37] investigated decreasing in PTH with increasing concentrations of 25-OH vitamin D and stated 2 point of intersections as 12 ng/mL and 28 ng/mL. In an other research 52 nmol/L vitamin D was determined as a threshold value which caused change in inclines of the regression line [38]. In a study Masud accepted PTH as a reference of 53 pg/dL and found a minimum level of 25-OH vitamin D level was 16.0±2.0 ng/mL that would keep PTH below 53 pg/dL. This study stated that the using of PTH could allow the identification of asymptomatic patients [7].

In our study the cut-off value of 25-OH vitamin D increased PTH above reference of ≥65 pg/mL was found as 18.5±0.38 ng/mL. Sourberbielle et al. [39] demonstrated that the normal upper limit for PTH was lowered from 65 pg/mL to 46 pg/mL in study which 25-OH vitamin D levels were above 30 nmol/L and underlined important of vitamin D in creating the reference range for serum PTH. Hyperparathyroidism is caused either from an internal or external abnormality induced production of PTH [40]. Calcium and 1,25-dihydroxyvitamin D have a negative feedback effect on PTH releasing [41]. Secondary hyperparathyroidism is characterized by high serum PTH level and hypocalcemia [42]. Primary hyperparathyroidism is diagnosed by assessment of clinical, laboratory and radiological findings [43]. In this study we evaluated laboratory data so we do not know about patient’s clinical and radiological findings. But we excluded data from oncology, organ transplant unit, nephrology, endocrinology and pediatrics department to minimize these effects. In our study we demonstrated a negative and positive correlation between calcium and PTH levels in patients with calcium 9.01–10 mg/dL and 10.01–11 mg/dL, respectively but we did not find a negative correlation in >11 mg/dL calcium groups. According to us this may be reflect that there were subjects with primary hyperparathyroidism. ALP level rises in vitamin D deficiency [44]. We found that ALP was higher in patients with 25-OH vitamin D level ≤30 ng/mL. ALP is a bone resorption marker, and associated with PTH levels [45]. Liu et al. [46] indicated that iPTH concentrations were meaningful increased in higher ALP quartiles. In our study ALP levels were higher in individuals with hyperparathyroidism than with non-hyperparathyroidism. Our result supported the relationship between parathormone and ALP.

The P level can be normal or low in vitamin D deficiency [47]. We invented that the P levels were lower in subjects with 25-OH vitamin ≤30 mg/dL compared to >30 mg/dL group. But there was no significantly difference. The limitations and incompleteness of our study are that the use of data from patients only admitted to the hospital, therefore factors such as sun exposure, diet, using of calcium and vitamin D supplement status are unknown.

We conducted our study in Antalya which is located in the Mediterranean region of Turkey. Although Antalya is sunny in most of year we found that majority of participants had no sufficient level 25-OH vitamin D. Only 22.6% patients have sufficient level. This result supports high ratio of 25-OH vitamin D deficiency and insufficiency in public. PTH may not increase in all patients with low vitamin D levels. Vitamin D levels were low in 87.7% of patients with hyperparathyroidism and majority of these patients had deficient or severe deficient level. For determining the threshold value of 25-OH vitamin D, serum PTH and 25-OH vitamin D levels should be interpreted together. PTH which plays important role in the adjustment of vitamin D should also be assessed for correct diagnosis of vitamin D insufficiency and deficiency. Using PTH as a parameter to assess vitamin D deficiency prevents missed abnormality.