Because of contradictory results, clinical significance of elevated levels of macroprolactin (macroprolactinemia) remains unclear. The aim of this study was to investigate whether macroprolactinemia determines levothyroxine action on hypothalamic-pituitary-thyroid axis activity and thyroid antibody titers in women with autoimmune hypothyroidism. The study population included 2 age-, body mass index-, hormone-, and thyroid antibody-matched groups of premenopausal women with untreated autoimmune subclinical hypothyroidism: 15 subjects with coexisting macroprolactinemia and 29 individuals with prolactin levels within the reference range. All included patients were then treated with levothyroxine for 6 months. Serum levels of thyrotropin, free thyroid hormones, prolactin and 25-hydroxyvitamin D, titers of thyroid peroxidase and thyroglobulin antibodies, as well as macroprolactin content were assessed at the beginning and at the end of the study. Except for 25-hydroxyvitamin D levels and macroprolactin content, there were no significant differences between both study arms in the investigated markers. All participants completed the study. In both treatment arms, levothyroxine treatment decreased thyrotropin levels, increased free thyroxine and free triiodothyronine levels, as well as reduced thyroid peroxidase titers, but this effect was less pronounced in women with macroprolactinemia. In women with normal prolactin levels, levothyroxine reduced also thyroglobulin antibody titers and increased 25-hydroxyvitamin D levels. In this group of patients, treatment-induced changes in hormone levels and thyroid antibody titers correlated with treatment-induced changes in 25-hydroxyvitamin D levels. The obtained results suggest that macroprolactin excess attenuates the impact of levothyroxine on hypothalamic-pituitary-thyroid axis activity and thyroid autoimmunity.© 2020, The American College of Clinical Pharmacology.