Introduction: In previous studies decrease in T3, '14, increase in T4, no change in T3, T4, TSH, FT4, FT3 was reported. Due to these controversies this study were done to determine the effect of Ramadan fasting on T3, T4, T3RU, TSH, FT3, FT4.Material & Methods: This study is a quasiexperimental (before- after) study &. was done on 90 male & 57 female volunteer religious students who were healthy & were fast for at least 25 days during Ramadan. Duration of fasting was 11.5 ± 0.5 hours. Blood samples were taken one week before and at one hour before sunset at 14th and 28th day of Ramadan. All lab assays was done by RIA method. And data analysis was done by SPSS (paired t Test and ANOVA).Results: Total T3 decreased significantly (157:: 27 to 127 ± 23, p<0.001). Total T4 decreased significantly too (8.66 ± 16 to 7.2 ± L1, p<0.001). FT3 increased in 14th day and then return to previous amounts (p<0.001). FT4 decreased significantly during Ramadan (1.36 ± 0.42 to 1.13 ± 0.34, p<0.001). TSH decreased mildly in normal range (1.28 ± 0.84 to 1.15±0.75, p<0.07). T3RU hasn't significant change (28.16 to 27.58, p<0.61).Conclusion: Regarding the above data it seems that thyroid function Tests (T4, T3, FT3, FT4) changes during Ramadan but this changes are within normal limit and are not only due to TEG or conversion effect because free forms of hormones are changed as well.