Abdollahpour, HamedKarimzadeh, MiladJafari Pastaki, NaghmehZamani, Hosseinali2026-06-252026-06-252026-062405-6650http://hdl.handle.net/10400.1/29143Thyroxine (T4), a key thyroid hormone, plays a crucial role in regulating growth, metabolism, and reproduction in fish, whereas methimazole (MMI), a thyroid peroxidase inhibitor, can disrupt these processes by inducing hypothyroidism. This study investigates thyroid-modulating compounds' physiological and reproductive effects on the growth and development of mature female goldfish (Carassius auratus). A total of 240 adult female goldfish were divided into four treatment groups: control (coconut oil), T4, MMI, and T4+MMI (combined). The fish were acclimatized for four weeks before receiving injections of the respective compounds. Growth performance, blood biochemistry, thyroid hormone (THs) levels, oocyte development, and liver histology were evaluated over a 28-day experimental period. Results indicated significant differences in growth indices, with the T4 group showing the highest weight gain, specific growth rates, and lowest feed conversion ratio (P < 0.05), while the MMI group exhibited the lowest growth parameters. Blood glucose, triglyceride, and total protein levels were significantly elevated in the T4-treated group, whereas cholesterol was reduced (P < 0.05). Plasma T3 and T4, were highest in the T4 group and lowest in the MMI group. Histological analysis revealed advanced oocyte maturation in the T4 group, with a higher proportion of oocytes at the O4 stage, while the MMI group showed delayed development, with most oocytes remaining at the O2 stage. The T4+MMI group exhibited intermediate effects, with some improvement in oocyte development relative to the MMI group. Hepatic histopathology demonstrated normal liver structure in the control and T4 groups, while severe hepatic alterations, including necrosis and vacuolation, were observed in the MMI group. The T4+MMI group displayed intermediate liver damage. These findings demonstrate that the pharmaceutical methimazole, an emerging aquatic contaminant, acts as a potent endocrine disruptor, inducing hypothyroidism that severely impairs growth, metabolic homeostasis, and reproductive maturation in fish. The partial mitigation by thyroxine suggests potential complex interactions in environments contaminated with multiple bioactive compounds. This study underscores the significant ecological risk of antithyroid drugs in aquatic ecosystems and contributes critical data for environ mental risk assessment.engGoldfishThyroid modulationEndocrine disruptionGonadal developmentLiver histopathologyjournal article10.1016/j.emcon.2026.100626