EFFECT OF EMPIRICAL VITAMIN D SUPPLEMENTATION ON THYROID-STIMULATING HORMONE LEVELS IN TREATMENT-NAÏVE PATIENTS WITH SUBCLINICAL HYPOTHYROIDISM

Abstract Aim: Subclinical hypothyroidism (SCH) is defined by elevated thyroid-stimulating hormone (TSH) with normal free thyroxine (fT4). Vitamin D deficiency may contribute to thyroid dysfunction through immune modulation and receptor activity in the thyroid gland. In Asian populations, where deficiency is highly prevalent, empirical supplementation may be more feasible than routine testing. This study assessed the effect of vitamin D replacement on TSH levels in treatment-naïve SCH patients. Methods: A randomized controlled trial was conducted at the Endocrinology Department, FGPC Hospital, Islamabad. Eighty patients aged 18–60 years with SCH (TSH 4.5–10 mIU/L, normal fT4) were randomized to receive either weekly oral vitamin D₃ (50,000 IU) for 12 weeks or standard care. Baseline and follow-up assessments included serum TSH, fT4, calcium, and phosphorus. Data were analyzed using SPSS v25; p<0.05 was considered significant. Results: Baseline characteristics (age, gender, BMI, thyroid profile, vitamin D, Anti-TPO, calcium) were comparable between groups (p>0.05). After 12 weeks, the vitamin D group showed a significant reduction in TSH compared with placebo (4.34 ± 0.68 vs. 6.67 ± 0.98 mIU/L, p<0.001). Serum 25(OH) vitamin D increased markedly in the intervention group (38.41 ± 5.32 vs. 15.41 ± 3.57 ng/mL, p<0.001). Free T4 and free T3 changes were minor and non-significant. Overall, 92.5% of vitamin D recipients achieved TSH reduction versus 15% in placebo (p<0.001). ANCOVA confirmed a significant treatment effect (F=152.207, p<0.001). Conclusion: Empirical vitamin D supplementation significantly lowers TSH in treatment-naïve SCH patients, supporting its role as a practical therapeutic strategy in populations with high deficiency prevalence.