Background/aim: Gestational transient thyrotoxicosis (GTT) is a transient, mild hyperthyroidism that occurs early in pregnancy and is due to human chorionic gonadotropin. There is no clear information about why only some pregnant women develop GTT. Previous papers stated that vitamin D plays a role in thyroid functions. We aimed to evaluate the relationship between vitamin D and GTT. Materials and methods: Fifty-three patients diagnosed with GTT at the 6th to 10th weeks of gestation were included in the study (GTT group). Thirty-five pregnant women with normal thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), and free thyroxine (fT4) levels served as a control group. Vitamin D, TSH, fT3, and fT4 levels were followed during entire the pregnancy. Results: TSH levels had been normalized at the 20th week of gestation in all patients with GTT (mean TSH: 0.56 ± 0.2 μIU/mL). Vitamin D levels were significantly lower in the GTT group than the controls (11.1 ± 7.7 and 16.5 ± 0.5 ng/mL, respectively; P = 0.008). Conclusion: Pregnant women who are diagnosed with GTT should be evaluated for possible vitamin D deficiency.