Selenium (Se), an essential component of deiodinases (DIOs), regulates the contents of thyroid hormones and thus improves animal growth. To explore the influences of selenium supplementation on fish growth metabolism, a total of 270 healthy grass carp (Ctenopharyngodon idella) were divided into three groups and feed three graded dietary selenium (0.141, 0.562, and 1.044 mg Se/kg) levels. The results showed that after 60-day feeding, dietary selenium improved the final body weight and specific growth rate (SGR) of grass carp. The hepatic DIO activities in selenium-supplemented groups were higher than those in control group. A significant increase in triiodothyronine (T3), free triiodothyronine (FT3), and thyroid-stimulating hormone (TSH) levels was accompanied by a decrease in the contents of thyroxine (T4) and free thyroxine (FT4) in selenium-supplemented groups. The histopathological observation of thyroid suggested that selenium deficiency resulted in hypertrophy of follicular epithelial cells. Moreover, the gene relative expression levels of dio1, dio2, and dio3 showed an increasing trend with the rising concentration of dietary selenium. The transcription levels of HPT axis-related genes (crh, tsh-β, ttr, tr-s, tpo, nis) and GH/IGF1-related genes (gh, ghr, igf1, igf1r) were significantly upregulated in selenium-supplemented groups. No significant differences in the above indicators were observed between 0.562 and 1.044 mg Se/kg diet group except T3 content and dio1 relative expression ratio. These results indicate that dietary selenium supplementation improves the hepatic DIO activities and thyroid hormone metabolism and regulates the transcription levels of HPT and GH/IGF axis-related genes, which may be responsible for the growth promotion in grass carp.

硒 (Se) 是脱碘酶 (DIO) 的重要组成部分，可调节甲状腺激素的含量，从而促进动物生长。为探讨补硒对鱼类生长代谢的影响，对270条健康草鱼（Ctenopharyngodon idella) 被分为三组，并饲喂三种分级的膳食硒 (0.141、0.562 和 1.044 毫克硒/千克)。结果表明，饲喂60天后，日粮硒提高了草鱼的最终体重和比生长率（SGR）。补硒组肝脏DIO活性高于对照组。在硒补充组中，三碘甲腺原氨酸 (T3)、游离三碘甲腺原氨酸 (FT3) 和促甲状腺激素 (TSH) 水平的显着增加伴随着甲状腺素 (T4) 和游离甲状腺素 (FT4) 含量的减少。甲状腺组织病理学观察提示缺硒导致滤泡上皮细胞肥大。此外，dio1、dio2的基因相对表达水平, dio3随着膳食硒浓度的升高而呈增加趋势。HPT轴相关基因（crh、tsh-β、ttr、tr-s、tpo、nis）和GH/IGF1相关基因（gh、ghr、igf1、igf1r）的转录水平在硒补充组中显着上调. 除T3含量和dio1外，0.562和1.044 mg Se/kg饮食组上述指标无显着差异相对表达率。这些结果表明，日粮补充硒可改善肝脏 DIO 活性和甲状腺激素代谢，并调节 HPT 和 GH/IGF 轴相关基因的转录水平，这可能是草鱼生长促进的原因。