BACKGROUND: Dendritic cells (DCs) are the most effective antigen-presenting cells and key regulators of immune response. The immunoregulatory properties of DCs strongly depend on the microenvironment in which DCs have been matured and activated. Thyroid hormones are an important part of this environment and regulate many vital processes including growth and cellular metabolism. The aim of the study was an analysis of the influence of thyroid hormones on blood DC subtypes ex vivo, including the surface expression of molecules involved in antigen presentation, costimulation, and maturation, as well as on functional properties of DCs in vitro. METHODS: Blood samples for the quantitative and phenotypic analysis of peripheral blood plasmacytoid and myeloid DC subtypes were collected from thyroidectomized patients at two time points: (i) at the time of the so-called stimulation with endogenous thyrotropin-a group of hypothyroid patients after l-thyroxine (L-T(4)) withdrawal (pretreatment group)-and (ii) after 2 months of L-T(4) administration for thyrotropin suppression-a posttreatment group. The phenotype of DCs including HLA-DR, costimulatory molecules (CD40, CD80, and CD86), and maturation marker CD83 was assessed by flow cytometry. The influence of isolated peripheral blood DCs on autologous peripheral blood mononuclear cell proliferation and cytokine secretion (interferon alpha, interleukin-12) under triiodothyronine (T(3)) deficiency or T(3) excess was investigated in culture experiments. RESULTS: The percentage of peripheral blood plasmacytoid and myeloid DCs was higher after L-T(4) administration when compared with the pretreatment group. Moreover, the expression of CD86 on both DC subtypes was higher in the L-T(4) treated than in the hypothyroid patients. In the in vitro experiments, T(3) stimulation increased CD86 expression on cultured DCs. The phenotypic difference was paralleled by enhanced ability of T(3)-stimulated DCs to activate interleukin-12 secretion and proliferation of autologous peripheral blood mononuclear cells (PBMLs) in coculture experiments. CONCLUSIONS: In the present study, we provide for the first time an evidence that the thyrometabolic status has an influence on the phenotype and function of human peripheral blood DCs. This observation may be of potential importance for the understanding of the pathogenesis of immune and endocrine disorders.