In this paper we review studies of brain cellular high-energy phosphate metabolism, as measured by phosphorus-31 magnetic resonance spectroscopy (31P-MRS), in subjects with major depressive disorder (MDD). We also review the literature on the role of thyroid hormones on the cellular high-energy phosphate metabolism in multiple organs. Finally, we review data on the efficacy of thyroid hormones as adjuvant treatment in MDD. The framework established by these findings enables us to hypothesize that dysfunction of brain cellular energy metabolism is a vulnerability factor for MDD, and that correcting cellular energy metabolism (e.g., with thyroid hormones) is a valid therapeutic strategy for improving the symptoms of MDD. We discuss our hypothesis in the context of other current theories on the mechanism of thyroid hormones in MDD.