Ortega-Rodríguez AC, Martínez-Hernández R, Monsiváis-Urenda A, et al. - This observational study with cases and controls was undertaken to evaluate the levels as well as the function of programmed cell death protein-1 (PD-1)⁺ natural killer (NK) cells in samples from autoimmune thyroid disease (AITD) patients. Participants were 40 patients with AITD, 16 with Hashimoto thyroiditis (HT), 24 with Graves’ disease (GD), and 15 healthy controls. GD patients were found to have raised levels of NK cells and the CD56^dimPD-1⁺ subset. An enhanced expression of the regulatory receptors NKG2A and NKG2C by CD56^brightPD-1⁺ NK cells was identified in HT. An elevated synthesis of IL-10 by CD56^brightPD-1^– NK cells was found in AITD patients, whereas an enhanced generation of interferon-γ was shown by CD56^dimPD-1⁺ cells from GD patients. There were significant links in patients with GD or HT between the concentrations of PD-1⁺ NK cells and clinical laboratory parameters. Findings demonstrated different abnormalities in NK cell subset levels, in PD-1 expression and its role in AITD patients, therefore, the complex role of these cells in this pathogenesis was further supported.