Objective: The aim of this study was to evaluate free thyroid hormone index (FTHI) (FT3 index/ FT4 index) as a parameter to use in differential diagnosis of gestational Graves’ disease (GGD) and gestational transient thyrotoxicosis (GTT). GGD and GTT are the most common causes of thyrotoxicosis in early pregnancy and differential diagnosis is important since they are treated differently. Method: Women who applied to our clinic with thyrotoxicosis were evaluated and 51 pregnant women were recruited to the study. Among the 51 women, 36 had GTT, 15 had GGD. The age, gestational age, thyroid stimulating hormone (TSH) levels, and FTHI (FT3 index / FT4 index) of all patients were recorded and compared. FT3/FT3 upper limit of normal (ULN) is referred as FT3 index. FT4 index is also referred as FT4/FT4 ULN. All diagnosis of GGD and GTT was confirmed with TRAb assays.Results: The mean values of FT3, FT3 index, FTHI of the GGD group were found to be statistically significantly higher compared to GTT group (p: 0.002, p: 0.001, 0.001; respectively). No statistically significant difference was found between groups in terms of age, gestational age, FT4, FT4 index and TSH (p:0.999, p:0.730, p:0.620, p:0.570, p:0.291; respectively) FTHI values were above 1 in all patients with GGD while it was below 1 in GTT group. Conclusions: The FTHI values above 1 may be used as a practical, cheap parameter for differential diagnosis between GGD and GTT.