Background and Aim: Hop with, scientific name of Humulus Lupulus, has got estrogenic, sedative, hypnotic, and antiseptic properties in the treatment of certain diseases. The present study aimed at assessing the effects of the herb on pituitary–thyroid hormones in adult male rats. Materials and Methods: This experimental study was conducted on 45 adult male Wistar rats. The rats were divided into 5 equal groups including control (no treatment), control (saline) and three experimental groups receiving doses of the hop flower extract 50,100, and 150mg/kg gavage for 10 days, respectively. At the end of the tenth day .,blood samples were derived from the rats’ hearts and their blood hormone concentrations of Thyrotropin (T3), Thyroxine (T4), and Thyrotropin (THS) were measured by means of gamma counter . The obtained data was analysed employing SPSS software (V: 18) using Anova and Tukey. Statistical tests at the significant level P≤0.05. Results: It was found that the experimental groups receiving doses of 150 and 100 mg/kg, their serum hormone levels of T3, T4 and TSH levels significantly reduced (P≥0.05). Conclusion: The results showed that the ethanol extract of hop flowers with a flavonoid phytoestrogens reduce T4, T3 and TSH hormones.