The interplay between thyroxine and cortisol in zebrafish: Effects on growth, physiology, and reproduction

In the current study, the role of thyroxine (T4) and cortisol (CO) on growth, reproduction, and whole-body hormone levels in male and female zebrafish was assayed. Fifty-seven days post-fertilization, zebrafish [132 females; initial weight: 0.3 ± 0.0 g and 132 males (initial weight: 0.2 ± 0.0 g)] were fed with diets including 10 mg T4 kg feed^-1 (T4), 10 mg CO kg feed^-1 (CO), 10 mg T4 kg feed^-1 + 10 mg CO kg feed^-1 (T4 + CO), and a control group (without hormones) for 28 days. At the end of the experiment, growth performance, feed utilization, whole-body hormone levels, gonadal development, and spawning performance were determined. The growth performance was improved following T4 administration in both males and females. Viscerosomatic and gonadosomatic indices in females were significantly enhanced in the T4 group compared with the other groups. Regarding oocyte developmental stages, a lower number of oocytes at final maturation were found in the CO group compared with the other treatments. Oocyte diameter was significantly increased in T4 compared with the other groups. Whole-body CO levels in both males and females were increased and decreased in CO and T4 treatments, respectively. Moreover, whole-body thyroid hormone levels in both sexes were higher in T4 group than the other groups. The hatching rate and survival rate at three days post-hatch were increased in the T4 group. Overall, the findings of this study underscore the stimulatory role of T4 in enhancing the physiological performance of zebrafish, leading to improved growth and increased reproductive output in both males and females.