Anemia is a frequent, although often underestimated, clinical condition accompanying thyroid diseases. Despite the fact that anemia and thyroid dysfunction often occur simultaneously, the causative relationship between the disorders remains ambiguous. Thyroid hormones stimulate the proliferation of erythrocyte precursors both directly and via erythropoietin production enhancement, while iron-deficient anemia negatively influences thyroid hormone status. Thus, different forms of anemia might develop in the course of thyroid dysfunction. Normocytic anemia is the most common, while macrocytic or microcytic anemia occurs less frequently. Anemia in hypothyroidism might result from bone marrow depression, decreased erythropoietin production, comorbid diseases, or concomitant iron, vitamin B12, or folate deficiency. Altered iron metabolism and oxidative stress may contribute to anemia in hyperthyroidism. The risk of anemia in autoimmune thyroid disease (AITD) may be related to pernicious anemia and atrophic gastritis, celiac disease, autoimmune hemolytic syndrome, or rheumatic disorders. The coexistence of anemia and thyroid disease constitutes an important clinical problem. Thus, the aim of this review was to provide a comprehensive summary of data on the prevalence, potential mechanisms, and therapy of anemia in the course of thyroid diseases from the clinical and pathogenetic perspectives. Thyroid dysfunction and AITD should be considered in a differential diagnosis of treatment-resistant or refractory anemia, as well as in the case of increased red blood cell distribution width. Of note, the presence of AITD itself, independently from thyroid hormone status, might affect the hemoglobin level.