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.