DIO2 and depressive symptom measurement: Importantly, in WKY rats, despite the lack of changes in the level of thyroid hormones in the blood, in the frontal cortex, we showed a decrease in the level of T3, the main thyroid hormones receptor (TRα), and the level of deiodinase 2, which confirms the theory that in depression the activity of thyroid hormones is weakened in the brain.