For more than 40 years thyroid hormones and mood disorders have been associated. Some psychiatric symptoms are produced by thyroid illnesses and there is a frequent association of thyroid dysfunction with mood disorders. Therefore, routine thyroid function assessment in patients with mood disorders and the treatment of sub-clinical thyroid dysfunctions is recommended. The usefulness of adding thyroid hormones to antidepressive treatment in euthyroid patients to obtain a potentiation effect has been probed repeatedly. The most common strategy is potentiation with T3, but high doses of T4 have been also used in patients with resistant depression. Thyroid hormones exert their action in the central nervous system through a variety of mechanisms: modulation of gene expression of several groups of proteins, some of them with known physiopathological implications in mood disorders and the influence over serotonin and noradrenergic neurotransmission, known to be one of the modes of action of antidepressants. Finally, it is also important to stress the complex relationship between psychiatric drugs, deiodinases and thyroid hormones, that can potentially help to understand the mechanisms of action of these drugs