In 1896 Riedel described a chronic sclerosing thyroiditis, occurring especially in women, that tends to progress inexorably to complete destruction of the thyroid gland and frequently causes pressure symptoms in the neck (97-99). It is exceedingly rare. In the Mayo Clinic series it occurred approximately on-fiftieth as frequently as Hashimoto's thyroiditis. It is approximately twice as frequent in men as in women and is found most often in the 30- to 60- year age group. The thyroid gland is normal in size or enlarged, usually symmetrically involved, and extremely hard. Occasionally involvement may be unilateral. Diagnostic confustion with sarcoma of the thyroid region has been reported (101). On pathologic examination the gland is replaced by dense fibrosis in which are scattered solitary follicular cells and occasional acini with small amounts of colloid. The fibrosis binds the thyroid firmly to the trachea and the strap muscles, from which it can be separated only with the greatest difficulty (ligneous thyroiditis) (102). The fibrosis may compress the trachea or esophagus. The disease may remain stable over many years, or it may progress slowly and produce hypothyroidism. Dyspnea, dysphagia, hoarseness, and aphonia are caused by the local pressure, and if there is enough pressure on both recurrent laryngeal nerves, there may be stridor. Sometimes the disease is asymptomatic and discovered only incidentally. The pathologic process may advance to complete replacement of the gland, and then symptoms and signs of hypothyroidism appear. Involvement of the parathyroid glands by the fibrotic process may result in hypoparathyroidism (103-107). Rarely, Riedel's thyroiditis may be associated with similar fibrosclerotic processes in other areas, including the lacrimal glands, orbits (108), parotid glands, mediastinum, lung, myocardium, retroperitoneal tissues, and bile ducts in varying combinations in the syndrome of multifocal fibrosclerositis (109). Fluorine-18 fluorodeoxyglucose [FDG] positron emission tomographic images have shown metabolic activity in an abdominal mass and increased glucose metabolism Iin the thyroid, probably resulting from active inflammation involving lymphocytes, plasma cells and fibroblant proliferation (110). FDG metabolic activity can also be used to assess a patient's response to therapy (111). This mixture of inflammatory cell infiltrate and fibrosis can also be visualised using dynamic magnetic resonance imaging with gadpentate dimeglumine (112) and appropriate T1- and T2- weighted images (113). Subcutaneous fibrosclerosis has also been noted but it is very rare (114). The occurrence of cerebral sinus thrombosis suggests that Riedel's thyroiditis may cause venous stasis, vascular damage, and possibly hypercoaguability (115). The results of laboratory tests of thyroid function are usually normal, but about one-third are hypothyroid. The erythrocyte sedimentation rate is not elevated, as in subacute thyroiditis, and there is no leukocytosis. Antithyroid antibodies are present in 67% of reported cases (99) and a mixed population of B- and T-cells is present in the thyroid, suggesting an autoimmune etiology or association. The B cell proliferation has been shown to be polyclonal (100). The occurrence of marked tissue eosinophilia and the extracellular deposition of eosinophil granule major basic protein suggests a role for eosinophils and their products in the development of fibrosis in Riedel's thyroiditis (116). Fibrosis may also be related to the action of TGF beta 1, as seen in murine thyroiditis (117). The manifestations of Riedel’s thyroiditis can be confusing. A patient was recently reported who, over 18 months after biopsy proven Riedel’s thyroiditis, developed hyperthyroidism, spontaneous primary hypoparathyroidism, acute compressive neck symptoms requiring emergency isthmusectomy, vocal cord paralysis, syncopal-like episodes, and Horner’s syndrome due to compression of the right carotid sheath. This patient is under therapy with glucocorticoids and tamoxifen (106).
Although there is no specific therapy for Riedel's thyroiditis, several management strategies are available dependent on the clinical features of the disease in the individual patient. Corticosteroid therapy has been found to be effective in some cases (109,118-122), probably those with active inflammation. Initial doses of up to 100mg per day of prednisolone have been used but sustained improvement has been reported with lower doses of 15-60 mg per day (109). There are no controlled trials of steroid therapy in Riedel's and although some patients obtain long term benefit after steroid withdrawal (123) others may relapse (124). The reasons for this variation are unclear but inflammatory activity and duration of disease may be relevant factors. In those who fail to respond to steroid therapy or relapse after withdrawal tamoxifen therapy should be tried. Three reports have described an encouraging response with this agent, admittedly in only a small number of patients (125,126, 127). It is possible that tamoxifen acts in Riedel's by inhibition of fibroblast proliferation through the stimulation of TGF beta. Combination therapy with prednisolone and tamoxifen is also effective (128).
As hypothyroidism is rare in Riedel's, thyroxine therapy is usually not required and is not thought to influence the course of the disease. Surgical intervention may be necessary to release the trachea or perform tracheotomy in the case of severe stridor. Unilateral lobectomy has been performed for unilateral disease (129) and larger resections should be considered in some instances. It is recommended that surgical exploration and biopsy are usually required to exclude malignancy which may be suspected at presentation.