The relationship between Vitamin D deficiency and thyroid function in the first trimester of pregnancy

Aims and Objective: There are few studies about the relationship between vitamin D deficiency and thyroid hormone levels in pregnancy. This study aimed to assess the relation serum vitamin D levels with thyroid hormones in the first trimester of pregnancy in Iran. Methods: In this case-control research 430 pregnant women (215 mothers with vitamin D deficiency, and 215 without this deficiency) attending prenatal clinics in Tehran, Iran were studied. 25 hydroxy vitamin D levels and thyroid function tests [free tetraiodothyronine (FT4), thyroid stimulating hormone (TSH), and free triiodothyronine (FT3)] were measured in all mothers. Data were analyzed using SPSS version 22 software. Kolmogorov-Smirnov test was used for normality testing of continuous variables; Student T-Test, and Mann-Whitney U test were used to compare the continuous parametric and nonparametric variables respectively in the 2 study groups; Linear regression model was used to assess the effect of potentially effective variables besides vitamin D on  thyroid function tests. Results: In Mann-Whitney U test performed, T4 levels (mean± SD) were significantly higher in vitamin D deficiency group compared with the control group (20.8 ±32.5 vs.14.4 ± 24.1 ng/dl, P-value: 0.04), but there were no significant differences between the 2 study groups regarding TSH, and free T3 levels. In Linear regression analysis, assessing the effects of vitamin D deficiency plus maternal age, Body Mass Index, and number of pregnancies on thyroid function tests, it was shown that vitamin D deficiency had only a significant direct relation with free T4 levels (β: 0.122, P-value: 0.01), but had no significant relations with the other thyroid function tests namely TSH, and free T3. Conclusion: A significant direct relationships were observed between Vitamin D deficiency with the level of thyroxin (T4) during early pregnancy. higher levels of T4 in vitamin D deficient mothers. Due to the negative effects of Vitamin D deficiency and thyroid hormones on the mother and fetus, further studies should be conducted which may help in more accurately screening during pregnancy.