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Vitamin D as a main immunomodulator plays an important role in a number of metabolic and autoimmune diseases. According to research studies, concentration of vitamin D can positively affect the thyroid gland function. Aim: To investigate the effect of vitamin D oral supplementation in plasma levels of TSH (thyroid stimulating hormone) and anti-thyroid antibodies (anti-TPO και anti-TG), as well as other related markers, in HT (Hashimoto Thyroiditis) patients with vitamin D insufficiency. Another aim of the present study was to assess information about vitamin D and its health related role in a pool of HT patients using an internet based questionnaire. Methods: In total 10 HT patients with vitamin D insufficiency were recruited (7 women; 3 men). Prior to vitamin D supplementation (time 0), blood was withdrawn from all patients to assess levels of vitamin D, TSH, Anti-TPO, Anti-TG. Additionally, patients were requested to fulfill a disease related questionnaire, while anthropometric indices namely body weight, height, waist circumference and hip circumference were also measured. Oral vitamin D supplementation (25000 I.U./week) was implemented for a total period of three months. After the end of intervention, the same blood tests were repeated. Statistical analysis was conducted using Microsoft Excel and IBM SPSS Statistics 25 program. Results: The results of the intervention showed a statistically significant decrease of TSH at 3 months compared to time 0 (4,64±3,3 μIU/mL vs. 2,6±0,94 μIU/mL respectively, p=0,037), meaning that TSH was reduced by 2 units. Non significant reductions were observed for Anti-TPO (reduced by 72,6 IU/mL, p=0,241) and Anti-TG levels (reduced by 115,5 IU/mL, p=0,285). In regards to anthropometry, body mass index (p=0,011) was reduced by 0.7 kg/m2 . During the study a total number of 4 patients did not receive medical treatment (T4, thyroxin). However, at the end of the intervention period these patients showed decrease (non significant) of TSH (reduced by 0,6 μIU/mL, p=0,068) Anti TPO(reduced by 203,8 IU/mL p=0,068) and Anti-TG (reduced by 315,8 IU/mL,p=0,068). Thus supporting the hypothesis that vitamin D could be an independent regulator of thyroid function. Regarding the internet based questionnaire, HT patients who received vitamin D supplementation had lower levels of TSH (11,47±13,11 μIU/mL) compared to those patients who did not take vitamin D supplementation (17,33±31 μIU/mL). Last but not least, while 78% of HT patients had vitamin D supplementation, only 18% had sufficiency (>30ng/ml). Conclusion: It is of vital importance that patients with HT should undergo blood test to assess vitamin D status. Oral supplementation of cholecalciferol by these patients could possibly lead to beneficial effects on thyroid function.