Riedel's thyroiditis: clinical, pathological and imaging features.

Riedel's thyroiditis (RT) is an extremely rare form of chronic thyroiditis, characterised by a fibroinflammatory process which partially destroys the thyroid, often involving surrounding tissues. We describe an unusual case of RT in a 38-year-old woman, and discuss its typical clinical, pathological, ultrasound, radionuclide scanning and magnetic resonance findings. We conclude that the diagnosis of RT is highly suggestive in the presence of a hard gland (or nodule), fixed to adjacent structures; 'cold' on Tc99 scan; hypoechoic with absence of vascular flow, invading the adjacent neck structures on ultrasound; hypointense in T1- and T2-weighted MR images; and showing fibrous tissue fragments with inflammatory cells at cytological examination. Thyroidectomy has to be performed to confirm the diagnosis and to relieve pressure symptoms.