Association Between Season of Birth and External Risk Factors in Patients With Hashimoto's Thyroiditis and Non-autoimmune Hypothyroidism.

Background Both Hashimoto's thyroiditis and non-autoimmune hypothyroidism are highly prevalent thyroid disorders. While their clinical presentations overlap, they are driven by distinct pathophysiological mechanisms and potentially influenced by different genetic susceptibilities and environmental triggers, including prenatal solar exposure and vitamin D status. This study aimed to evaluate and compare the season of birth, serum vitamin D levels, body mass index (BMI), smoking habits, and family history of thyroid diseases between patients with Hashimoto's thyroiditis and patients with non-autoimmune hypothyroidism in Tuzla Canton. Materials and methods This retrospective observational cross-sectional study reviewed the medical records of 200 patients evaluated and followed up at the Department of Nuclear Medicine and Radiology of a tertiary care university hospital in Bosnia and Herzegovina. Participants were equally divided into two groups: patients with Hashimoto's thyroiditis (n = 100) and patients with non-autoimmune hypothyroidism (n = 100). Numerical variables were analyzed using the Mann-Whitney U and Kruskal-Wallis tests, while categorical variables were evaluated using the χ² test. Post hoc pairwise comparisons were performed with Bonferroni correction. Results The study population comprised 189 women (94.5%) and 11 men (5.5%). The distribution of birth seasons did not differ significantly between the groups (χ² = 2.444, df = 3, p = 0.485). Similarly, no statistically significant differences were observed in median serum vitamin D levels (p = 0.931) or smoking rates (23.0% in the Hashimoto's group vs. 22.0% in the hypothyroid group, p = 0.866). However, patients with non-autoimmune hypothyroidism were significantly older (median: 59.0 vs. 49.0 years, p < 0.001) and presented with significantly higher BMI values (median: 28.20 kg/m² vs. 26.21 kg/m², p = 0.008) compared to those with Hashimoto's thyroiditis. A positive family history of thyroid disease was significantly more frequent among patients with Hashimoto's thyroiditis than those with hypothyroidism (62.0% vs. 47.0%, χ²= 4.537, df = 1, p = 0.033). Within the non-autoimmune hypothyroidism group, vitamin D levels differed significantly across BMI categories (p = 0.030), with overweight patients exhibiting significantly lower median vitamin D levels compared to normal-weight patients (median: 18.50 vs. 24.00, adjusted p = 0.045). Conclusion Season of birth, smoking status, and overall serum vitamin D levels do not distinguish Hashimoto's thyroiditis from non-autoimmune hypothyroidism. However, patients with non-autoimmune hypothyroidism present with higher BMI, and those who are overweight are particularly prone to lower vitamin D levels. In contrast, Hashimoto's thyroiditis is more strongly characterized by a positive family history of thyroid disease, highlighting a more pronounced hereditary component.