Riedel thyroiditis, an uncommon form of chronic thyroiditis in which the thyroid gland is replaced by fibrous tissue, is difficult to differentiate from the fibrous variant of Hashimoto thyroiditis or lymphoma. We report a patient illustrating this. A 54 year old lady with a long standing goitre and 3 year history of stable hypothyroidism was referred for progressive thyroid gland enlargement over several months, dysphagia and an increasing thyroxine requirement. Clinically, she had a diffusely hard but mobile, nodular goitre with retrosternal extension and positive Pembertons sign. Laboratory investigations showed hypothyroidism (TSH 45.4 mIU/l, free T4 11.4 pmol/l), raised TPO titre (>600 IU/ml) and ESR 57 mm/h. Ultrasound thyroid showed a diffusedly enlarged thyroid, with replacement by markedly inhomogenous hypoechoic masses with lobulated contours, minimal increase in vascularity and few foci of calcifications. Some enlarged neck nodes with loss of fatty hilum. CT scan confirmed a diffusely enlarged heterogenous thyroid with tracheal narrowing (1.1 cm diameter) and level II, III nodes (1.2 cm or less). FNA thyroid was compatible with lymphocytic thyroiditis but with atypical lymphoid cells, also seen on the FNA neck node. A thyroid Tru-Cut biopsy excluded lymphoma but revealed lymphoid aggregates and lymphoplasmacytic infiltrate in a sclerotic background raising the possibility of Riedels or late stage Hashimotos thyroiditis. Systemic fibrosis was excluded. At the thyroid multidisciplinary team meeting, the case was discussed, decision made for conservative management of possible Riedels thyroiditis. The patient was commenced on a tapering course of systemic steroids with optimisation of thyroxine replacement. Within weeks, she improved significantly clinically with normalisation of her thyroid function and ESR. A repeat CT scan showed reduction in size of the thyroid with unchanged appearances of her neck nodes. She remains on low dose steroids that we propose to taper subsequently.