Association of vitamin D status with ferritin and TSH levels across age and sex groups: a cross-sectional study

Vitamin D deficiency has been implicated in disturbances of thyroid function and iron metabolism, but data from Iraqi clinical populations remain limited. We performed a hospital-based cross-sectional study (August 2024-January 2025) of 99 adults (15-45 years; 49 males, 50 females) attending Mosul General Hospital to examine associations between serum 25-hydroxyvitamin D status and two biochemical markers: thyroid-stimulating hormone (TSH) and ferritin. Participants were classified as vitamin D deficient (≤10 ng/mL, n = 37), insufficient (11-20 ng/mL, n = 31), or sufficient (>20 ng/mL, n = 31). Mean TSH was highest in the deficient group (5.09 ± 0.77 μIU/mL) and lowest in the sufficient group (1.90 ± 0.13 μIU/mL; p < 0.001). Mean ferritin was lowest in the deficient group (42.8 ± 3.07 ng/mL) and higher in the sufficient group (60.26 ± 7.3 ng/mL; p = 0.03). Correlation analysis showed a significant inverse association between vitamin D and TSH (r = -0.488, p < 0.001) and a positive association between vitamin D and ferritin (r = 0.212, p = 0.035). These findings indicate that lower vitamin D status is associated with higher TSH and lower iron stores in this hospital cohort. Consideration of vitamin D and iron status may aid the interpretation of abnormal TSH values and warrants prospective investigation to test whether nutritional correction modifies thyroid-related outcomes.