However, the co-occurrence of hypothyroidism and depression led to changes in the presence or abundance of some metabolic markers, such as decreased pyruvate dehydrogenase levels in the frontal cortex, decreased lactate levels in the cytosolic fraction of the hippocampus, increased PDK4 expression in the hippocampus, and decreased ETS and OXPHOS coupling efficiency. Here, PDK4 is linked to major depressive disorder.