The aims of this study were to evaluate: (1) associations of vitamin D with the presence/severity of Hashimoto’s thyroiditis (HT) and (2) correlations of vitamin D with thyroid-related phenotypes. Total 25(OH)D (vitamin D in the text) was measured from stored serum samples of 461 HT patients and 176 controls from a Croatian Biobank of HT patients (CROHT). (1) Vitamin D levels, and proportions of vitamin D deficiency, were compared between HT cases and controls. HT patients were additionally divided into two groups (MILD and OVERT) to take into account HT severity. (2) Correlations between vitamin D and 10 clinical phenotypes in all HT patients and two subgroups of HT patients were tested using the Spearman correlation test. Our analyses were adjusted for age, gender, BMI, smoking status and seasonality of blood sampling. (1) No significant differences in vitamin D levels, or proportions of vitamin D deficiency, were detected between HT patients of all disease stages and controls. However, a nominally significant difference in vitamin D levels between MILD and OVERT subgroups (OR = 1.038, p = 0.023) was observed. Proportions of individuals with vitamin D deficiency during winter–spring were high: all HT cases (64.69%), MILD (60.64%), OVERT (68.7%), controls (60.79%). (2) A nominally significant negative correlation between vitamin D and TSH in all HT patients (r = −0.113, p = 0.029) and a positive correlation between vitamin D and systolic blood pressure in OVERT HT patients (r = 0.205, p = 0.025) were identified. Our study indicates that there is no association between vitamin D and HT; however, there may be a subtle decrease in vitamin D levels associated with overt hypothyroidism.

Objective: We evaluated correlations of occupational physical activity (OPA) and recreational exercise (RE), respectively, with thyroid function in patients with Hashimoto’s thyroiditis (HT). Methods: We included 438 individuals with clinically diagnosed HT. Information on OPA and RE were collected through a self-report questionnaire. We assessed correlations between clinical phenotypes (TSH, T3, T4, fT4, TgAb, TPOAb, thyroid volume, vitamin D) and physical activities (OPA and RE) in all HT patients (ALL) and in two severity-based subgroups of patients (MILD and OVERT). Results: The main novel findings are significant correlations between increase in OPA and (i) a decrease in fT4 (OVERT, r = −0.265, p = 0.0002 and ALL, r = −0.138, p = 0.006); (ii) an increase in TSH (ALL, r = 0.124, p = 0.014 and OVERT, r = 0.183, p = 0.013) and (iii) an increase in TPOAb antibodies (ALL, r = 0.101, p = 0.045). In contrast, we observed correlations between increase in RE and: (i) a decrease in TSH (OVERT, r = −0.238, p = 0.001); (ii) a decrease in TgAb antibodies (OVERT, r = −0.194, p = 0.01) and (iii) an increase in vitamin D levels (ALL, r = 0.146, p = 0.005 and OVERT, r = 0.173, p = 0.023). Conclusions: Our results suggest that, unlike RE, OPA correlates with decreased thyroid function and increased thyroid autoimmunity. Our study proposes that the PA health paradox also applies for the thyroid health.

In this cross-sectional observational study, we investigated whether recreational exercise (RE) influences systemic inflammation in Hashimoto’s thyroiditis (HT) across different disease severity groups. We analyzed 403 participants from the Croatian Biobank of Patients with HT (CRO-HT), including 173 controls and 230 HT patients (euthyroid, levothyroxine [LT4]-treated, and hypothyroid). Serum levels of 92 inflammatory proteins were measured using the Olink^® Target 96 Inflammation panel, and exercise status was assessed via structured questionnaires. Linear regression revealed distinct protein associations depending on thyroid status. In controls, RE was associated with reduced MMP-10 and FGF-5, reflecting cardiovascular and muscle benefits. In euthyroid patients, RE was associated with decreased CXCL9 and TRAIL, implicating reduced type 1 inflammation and vascular risk. LT4-treated patients showed increases in IL-15RA and IL-24 with RE, suggesting improved muscle metabolism and anti-inflammatory effects. In hypothyroid patients, RE was associated with reduced CCL20 and increased HGF, while changes in TRANCE and TWEAK indicated mixed effects on bone and immune regulation. Notably, RE was associated with reduced CXCL9 and CCL20, two proteins previously linked to HT risk. Overall, RE is associated with distinct changes in inflammatory profiles across HT disease severity groups, with the most favourable responses observed in LT4-treated patients, suggesting synergy with hormone therapy.

We investigated whether recreational exercise (RE) influences systemic inflammation in Hashimoto’s thyroiditis (HT) across different disease severity groups. We analyzed 403 participants from the Croatian Biobank of Patients with HT (CROHT), including 173 controls and 230 HT patients (euthyroid, levothyroxine [LT4]-treated, and hypothyroid). Serum levels of 92 inflammatory proteins were measured using the Olink® Target 96 Inflammation panel, and exercise status was assessed via structured questionnaires. Linear regression revealed distinct protein associations depending on thyroid status. In controls, RE was associated with reduced MMP-10 and FGF-5, reflecting cardiovascular and muscle benefits. In euthyroid patients, RE was associated with decreased CXCL9 and TRAIL, implicating reduced type 1 inflammation and vascular risk. LT4-treated patients showed increases in IL-15RA and IL-24 with RE, suggesting improved muscle metabolism and anti-inflammatory effects. In hypothyroid patients, RE was associated with reduced CCL20 and increased HGF, while changes in TRANCE and TWEAK indicated mixed effects on bone and immune regulation. Notably, RE was associated with reduced CXCL9 and CCL20, two proteins previously linked to HT risk. Overall, RE is associated with distinct changes in inflammatory profiles across HT disease severity groups, with the most favourable responses observed in LT4-treated patients, suggesting synergy with hormone therapy.