Associations Between Serum 25-Hydroxyvitamin D Levels and Thyroid Function Parameters in Previously Healthy Children Aged 6 to 24 Months

Introduction

Thyroid diseases, including euthyroid goiter, hypothyroidism, and hyperthyroidism, are estimated to affect 1.4–6% children worldwide.¹ Normal thyroid hormone levels play a key role in maintaining normal brain development, height, and growth velocity. Untreated overt thyroid diseases before the age of 3 years is well known to lead to a short stature, mental retardation, and brain damage.^2,3

Testing of vitamin D levels has substantially increased in recent years, and emerging evidence suggests that vitamin D insufficiency is a common condition worldwide.^4,5 Vitamin D insufficiency, defined as 25-hydroxyvitamin D [25(OH)D] level < 75 nmol/L, is estimated to affect 40% of the population,⁶ and it might be a novel risk factor for some chronic diseases. Consequently, many studies have disclosed the possible role of vitamin D in non-skeletal clinical outcomes, including autoimmune diseases, infectious diseases, metabolic syndromes, and cardiovascular disorders.^7,8 Furthermore, vitamin D plays an important role in immunomodulation, potentially affecting the thyroid gland through immune-mediated processes.⁹ Recent evidence has demonstrated that the distribution of thyroid stimulating hormone (TSH) level is influenced by 25(OH)D levels. A large population-based study reported that increased 25(OH)D levels were associated with decreased circulating TSH among younger individuals (mean age 29.2 years).¹⁰ Naghmeh et al suggest that a serum 25(OH)D level > 125 nmol/L is required for the maintenance of euthyroid functions.¹¹ Nevertheless, the effect of 25(OH)D level on thyroid functions is still controversial. A population-based study using data collected by the National Health and Nutrition Examination Survey in the USA found that 25(OH)D levels were associated with neither thyroid functions nor thyroid antibody status among subjects aged 12–85 years.¹²

Few studies have focused on investigating the correlation between 25(OH)D levels and thyroid disorders in children. In one such study, the group of children diagnosed with Hashimoto’s thyroiditis had a significantly higher prevalence of vitamin D deficiency than the control group.¹³ A similar study found a significant association between low vitamin D levels and autoimmune thyroiditis in Egyptian children; moreover, the study also reported that serum 25(OH)D levels were negatively associated with TSH and thyroid autoantibody levels.¹⁴

There is a scarcity of data on the effect of vitamin D status on thyroid function in children. Based on a large database of healthy children with detailed data on 25(OH)D status and thyroid hormone levels, we aimed to investigate the association between 25(OH)D status and thyroid function, and further, to assess the relationship between 25(OH)D levels and thyroid function parameters.

Patients and Methods

Study Design and Participants

The data for the cross-sectional study were extracted from a hospital information system database, which was constructed to include all routine health check-up data of children to promote early childhood development at Guangdong Women and Children’s Hospital. In the present analysis, we included 6–24 month-old children with data of serum 25(OH)D, TSH, free triiodothyronine (FT3), and free thyroxine (FT4) levels from January 2015 to May 2017. Additionally, the following information was available: age, gender, and season of the testing. To ensure a relatively healthy study population, children with a history of illness (including clinical thyroid dysfunctions, skeletal diseases, genetic syndromes, and malabsorptive disorders) were excluded. After considering the inclusion and exclusion criteria, 2869 children aged 6–24 months were included in the final analysis.

Ethics Statement

Ethical approval was obtained from the Institutional Review Board of Guangdong Women and Children’s Hospital, Guangzhou, China (No. 201,701,068). This study was conducted in accordance with the Declaration of Helsinki. The requirement for written informed consent was waived owing to the retrospective nature of the study. The privacy of the participants was preserved with anonymized data.

Laboratory Assessments

Serum 25(OH)D levels were measured by electrochemiluminescence immunoassay. The detailed laboratory procedures have been reported elsewhere.¹⁵ Serum 25(OH)D levels were divided into three categories: deficiency (25(OH)D < 50 nmol/L), insufficiency (50 nmol/L ≤ 25(OH)D < 75 nmol/L), and sufficiency (25(OH)D ≥ 75 nmol/L) according to the guidelines released by the Endocrine Society,¹⁶ and TSH, FT3, and FT4 levels were measured by electrochemiluminescence immunoassay. The reference ranges for thyroid function parameters were as follows: TSH, 0.35–4.94 mIU/L; FT3, 3.69–7.68 pmol/L; and, FT4, 11.84–22.52 pmol/L. Thyroid dysfunctions were defined as: hyperthyroidism (TSH < 0.35 mIU/mL, FT3 > 7.68 pmol/L and FT4 > 14.4 pmol/L), subclinical hyperthyroidism (TSH < 0.35 mIU/mL, normal FT3 and FT4 levels), hypothyroidism (TSH > 4.94 mIU/mL, FT3 < 3.69 pmol/L and FT4 < 11.84 pmol/L), and subclinical hypothyroidism (TSH > 4.94 mIU/mL, normal FT3 and FT4 levels).

Statistical Analysis

The means and standard deviations were used to describe the continuous variables. Categorical variables were presented as frequencies and proportions (%). The seasons of blood sample testing were categorized as follows: March to May, spring; June to August, summer; September to November, autumn; and, December to February, winter. For comparison of multiple groups, the ANOVA test was used for continuous variables, and the chi-square test was used for categorical variables. To reduce skewness and kurtosis, a Blom transformation was applied to TSH, FT3, and FT4, and 25(OH)D level values. Pearson correlation analyses were conducted to determine the associations between Blom-transformed TSH, FT3, and FT4, and 25(OH)D levels. Adjusted odds ratios (OR) and 95% confidence intervals (CI) for the associations of vitamin D deficiency or insufficiency with thyroid dysfunctions were estimated using logistic regression models. P < 0.05 was considered as statistically significant. All statistical analyses were performed using the SPSS statistical software package (V20, IBM Statistics, Chicago, IL, USA).

Results

The study population of 2869 children aged 6–24 months had a mean serum 25(OH)D level of 79.7 ± 28.1 nmol/L (Figure 1A) and 12.5% vitamin D deficiency and 36.0% insufficiency rates. The rate of vitamin D deficiency was lower in the group aged 6–12 months (8.6%) than in the group aged 13–24 months (15.3%). Of these, in the participants with no clinical symptoms, the rates of thyroid dysfunctions detected were as follows: 2.9%, hypothyroidism; 4.2%, subclinical hypothyroidism; 0.9%, hyperthyroidism; and, 1.7% subclinical hyperthyroidism. The participants’ characteristics are presented in Table 1.

Table 1 Participants’ Characteristics by 25(OH)D Levels in Children Aged 6–24 Months

Figure 1 Serum 25-hydroxyvitamin D [25(OH)D] level distributions (A) and correlations between thyroid stimulating hormone (B), free triiodothyronine (C), and free thyroxine (D), and serum 25(OH)D levels.

Serum 25(OH)D levels had no significant correlation with TSH (r = 0.022, P = 0.236), FT3 (r = 0.014, P = 0.440), and FT4 (r = −0.059, P = 0.056) levels (Figure 1B–D). On analyzing according to 25(OH)D-level quartiles (Q1: 10.5–60.2 nmol/L, Q2: 60.3–76 nmol/L, Q3: 76.1–95.3 nmol/L, and Q4: 95.3–280.8 nmol/L), the thyroid parameters (TSH, FT3, and FT4 levels) were also not significantly different between 25(OH)D level quartiles (all P > 0.05) (Figure 2).

Figure 2 Thyroid hormones with serum 25-hydroxyvitamin D [25(OH)D] quartiles. Thyroid stimulating hormone, free triiodothyronine, and free thyroxine did not significantly differ with 25(OH)D quartiles in children aged 6–24 months.

Table 2 shows the logistic regression analysis, which assessed the association between vitamin D deficiency or insufficiency and thyroid dysfunctions. Vitamin D deficiency [25(OH)D < 50 nmol/L] was associated with an increased risk of hypothyroidism (adjusted OR = 2.16, 95% CI: 1.18–3.94) after adjusting for age, sex, and season of testing, but not with subclinical hypothyroidism, hyperthyroidism, and subclinical hyperthyroidism. Further, the hypothyroidism subgroup was analyzed to evaluate the associations between serum 25(OH)D levels and thyroid-related measures. Serum 25(OH)D levels did not correlate with TSH (r = 0.042, P = 0.704), FT3 (r = 0.035, P = 0.751), and FT4 (r = 0.089, P = 0.421) levels. On analyzing the probable interactions between 25(OH)D levels and seasons and their influence on thyroid levels, no significant differences in thyroid hormone levels were found between the different quartiles of vitamin D levels (Table 3).

Table 2 Association Between Vitamin D Deficiency, Insufficiency, and Thyroid Dysfunction

Table 3 Comparison of Different Thyroid Parameters with 25(OH)D Quartiles and Seasons

Discussion

25(OH)D is the principal stored form of vitamin D. The measurement of serum 25(OH)D levels is considered to be the best diagnostic test to assess the vitamin D status.^17,18 Although the definition of vitamin D deficiency has remained controversial, serum 25(OH)D concentration < 50 nmol/L is usually defined as deficiency by most authors.¹⁶ According to the definition used in this cross-sectional study of children aged 6–24 months, our findings are in agreement with the findings of other reports that show a high prevalence of vitamin D deficiency and insufficiency.⁶

Of the children without overt thyroid diseases, 2.9% were detected with hypothyroidism. Statistically, a significant association was found between vitamin D deficiency and the risk of hypothyroidism. Our results indicate that vitamin D might also have an immunomodulating effect on thyroid functions in the pediatric population. However, no significant associations were observed for subclinical hypothyroidism, hyperthyroidism, and subclinical hyperthyroidism. The underlying mechanism of the link between vitamin D and autoimmunity is not fully understood; it might be related to its anti-inflammatory and immunomodulatory functions.¹⁹ Several studies have analyzed the supposed associations between vitamin D levels and Hashimoto’s thyroiditis, autoimmune thyroiditis, and Graves’ disease in adult subjects,^20–23 with inconclusive results. In most of these studies, patients with overt thyroid diseases had lower 25(OH)D levels than normal or control subjects. Furthermore, no strong association was found between vitamin D status and abnormal thyroid functions in large population-based studies.^10,24 The results of the present study, based on an early-childhood population, found that vitamin D deficiency was associated with an increased risk of hypothyroidism, after adjusting for age, sex, and season of testing. However, no associations were found between serum 25(OH)D levels and thyroid-related measures. The conflicting results may stem from cutoff values for hypothyroidism or vitamin D deficiency. Moreover, it implies that there may not be a linear relationship between serum 25(OH)D levels and thyroid-related measures (TSH, FT3, and FT4).

Some studies have found that serum 25(OH)D concentrations and thyroid hormone levels may be affected by seasonal variability.^25,26 Although the difference between FT3 and 25(OH)D quartiles was significant in the autumn subgroup (P = 0.037), the actual differences between FT3 levels were very small and the overall results tended to accentuate no difference. For this reason, we considered that this phenomenon is probably a chance finding and should not be regarded as conclusive. Inconsistent conclusions in different studies might be related to the study design, region, heterogeneity of the study population, sample collection seasons, thyroid hormones and vitamin D level detection methods, and evaluation criteria. More powerful research in the future is needed.

Our study adds to the lack of research between vitamin D status and thyroid functions in the early-childhood population. However, our results should be interpreted in the light of possible limitations. The effects of serum 25(OH)D levels on thyroid functions would be confounded by other factors. The data for the present study were extracted from the hospital information registration system. The feeding methods, vitamin D supplements, outdoor activities, and other confounding factors, which affect vitamin D levels, were not available in the registers. Therefore, the associations between serum 25(OH)D levels and thyroid functions might be underestimated. Moreover, there were a lack of clinical and biochemical data that would have allowed better sub-classification, such as TSH receptor antibody, anti-thyroglobulin, and anti-thyroid peroxidase antibody levels. Finally, our study is a single-center and cross-sectional study, which cannot be inferred for causality. The sample is not from the random general population, and the representativeness of the participants might be affected by selection bias.

Conclusion

A vitamin D deficiency is associated with hypothyroidism. No significant relationships were observed between 25(OH)D levels and TSH, FT3, and FT4 in children aged 6–24 months. Further studies are needed to support the associations and to determine whether vitamin D deficiency is a risk factor for the progression of thyroid disorders in early childhood.