Objectives: Gonadotropin-releasing hormone agonist (GnRHa) could influence the levels of sex hormones and thyroid hormones. The aim of this study was to investigate the effect of GnRHa on thyroid function. Materials and methods: The data of the patients were collected from the registrations of July 2014-October 2014. A total of 41 women who underwent one-time IVF cyclus were evaluated in this cross-sectional study. The patients were categorized into two groups according to the serum T-3, T-4, and TSH levels before and 2 weeks after the administration of GnRHa. Results: Mean basal TSH and mean TSH levels on hCG day were 1.98 +/- 0.77 and 1.75 +/- 0.70, respectively. The difference between the two groups was statistically significant (p < 0.05). GnRHa did not lead to statistically significant difference on serum-free T-3 and T-4 levels. Conclusions: In conclusion, our results demonstrate that GnRHa led to a decrease on serum TSH level. Serum-free T-3 and T-4 levels were remained unchanged and this might be due to early measurement of the hormone levels (just 2 weeks later from GnRHa administration).