From this systematic review of the literature to clarify the relationship between maternal thyroid changes and postpartum depression, based on our search criteria, it was observed that the studies on the subject are heterogeneous in terms of study size, population studied, design (prospective, case–control, transversal), psychometric scale, and the evaluation of thyroid hormones (different analysis methods and different cutoff points). The gene discussed is TG; the disease is postpartum depression.