Association between vitamin D levels and thyroid status; diagnostic insights into hyperthyroidism and hypothyroidism – A cross-sectional diagnostic study

Introduction: The correlation of vitamin D and thyroid function has been a growing area of interest in endocrinology, particularly regarding its potential diagnostic and therapeutic implications for hyperthyroidism and hypothyroidism. Objectives: This cross-sectional diagnostic study explores the correlation of vitamin D with thyroid health, offering insights into these interactions’ mechanisms and clinical relevance. Patients and Methods: This prospective cross-sectional diagnostic study was conducted on 98 patients at Al-Sader teaching hospital in Misan, Iraq, between October 2018 and March 2019. Demographic and clinical data were collected through interviews and hospital records, while serum samples were obtained for analysis of thyroid hormones (TSH, FreeT4 and FreeT3) and vitamin D levels. Patients were categorized as euthyroid, hypothyroid, or hyperthyroid based on TSH levels and the correlation between them with vitamin D levels was assessed using statistical tests. Results: The analysis revealed differing associations between vitamin D levels and thyroid status. For hyperthyroidism, a modest inverse correlation was observed, with an unadjusted odds ratio (OR) of 0.89 indicating an 11% reduced likelihood of hyperthyroidism per nanogram per milliliter (ng/mL) increase in vitamin D levels, though this was not statistically significant after adjustment (adjusted OR: 0.90). In contrast, a significant inverse relationship was found for hypothyroidism, with unadjusted and adjusted ORs of 0.68 and 0.62, respectively, showing that higher vitamin D levels reduced the likelihood of hypothyroidism by 32% and 38%, even after controlling for confounders. Diagnostic analysis using the area under curve (AUC) values showed moderate discriminatory ability for identifying non-hyperthyroidism (AUC: 0.715) with a sensitivity of 80% and specificity of 39% at a cutoff of 22.50 ng/mL. However, the diagnostic performance for non-hypothyroidism was excellent (AUC: 0.925), with a sensitivity of 85% and specificity of 97% at a cutoff of 20.00 ng/mL. Conclusion: The analysis suggests that vitamin D levels are significantly associated with a reduced likelihood of hypothyroidism; however, showed a modest significant association with hyperthyroidism only in the unadjusted analysis. Diagnostic performance was excellent for identifying non-hypothyroidism but moderate for non-hyperthyroidism, indicating a stronger role for vitamin D in hypothyroidism. These findings suggest that vitamin D may play a more substantial role in hypothyroidism than hyperthyroidism.