The present study describes in depth a case of Riedel thyroiditis (RT) to clarify its pathogenesis and its putative inclusion in the spectrum of IgG4-related disease. We report the clinicopathological, immunohistochemical, and ultrastructural features of a case of RT in a 39-year-old white Spanish woman, admitted with a hard goiter and cold nodule in the left thyroid lobe. This case represents 0.05 % of a series of 1,973 consecutive thyroidectomies performed in our hospital. More than 80 % of the left thyroid lobe was effaced by fibrosis and inflammation (lymphocytes, 57 IgG4+ plasma cells per 1 high-power field, an IgG4/IgG ratio of 0.67, and eosinophils) with extension into the surrounding tissues and occlusive phlebitis. Immunostaining for podoplanin (D2-40) detected signs of increased lymphangiogenesis in the fibroinflammatory areas that were confirmed by electron microscopy. A strong, diffuse stain for podoplanin and transforming growth factor ß1 was also detected in the same areas. The increased number of lymphatic vessels in RT is reported for the first time. Our findings support the inclusion of RT within the spectrum of IgG4-related thyroid disease (IgG4-RTD). Although the etiology and physiopathology of IgG4-RTD still remain elusive, the results obtained in the present case suggest the participation of lymphatic vessels in the pathogenesis of RT.