NK cells have an important role in innate immunity and in the regulation of immune response. The role of NK cells expressing the PD-1 regulatory receptor has not been explored in patients with autoimmune thyroid disease (AITD).To analyze the levels and function of PD-1 + NK cells in samples from AITD patients.Cases and controls, observational study.Hospital Universitario la Princesa, Spain.Forty patients with AITD, sixteen with Hashimoto thyroiditis (HT) and twenty-four with Graves' disease (GD), and fifteen healthy controls.Multi-parametric flow cytometry analysis of peripheral blood NK cells. In vitro assays of cytotoxic activity of NK cells, and synthesis of cytokines.Main outcome measures. Levels and function of PD-1 + NK cells in blood samples from AITD patients and controls.Increased levels of NK cells and the CD56 dimPD-1 + subset were observed in GD patients. In HT, an enhanced expression of the regulatory receptors NKG2A and NKG2C by CD56 brightPD-1 + NK cells was detected. AITD patients showed an increased synthesis of IL-10 by CD56 brightPD-1 - NK cells, whereas CD56 dimPD-1 + cells from GD patients exhibited an enhanced production of IFN-γ. PD-1 + NK cells from patients with GD and HT showed an increased cytotoxic activity. Significant associations were observed in patients with GD or HT between the levels of PD-1 + NK cells and clinical laboratory parameters.The different abnormalities in NK cell subset levels, in the expression of PD-1 and its function in AITD patients' further support the complex role of these cells in this pathogenesis.