Riedel fibrosing thyroiditis is a rare disease characterized by chronic inflammation and fibrosis of the thyroid gland. It is also known as Riedel struma, ligneous struma, or chronic invasive fibrous thyroiditis. Classically, it presents as hypothyroidism with a "stone-like, hard-as-wood" non-tender thyroid gland.
Over time, the thyroid parenchyma and surrounding structures are replaced with dense fibrous tissue, leading to the destruction of the follicular cells of the thyroid gland. The fibrosis then extends to involve local structures, such as the airways. This causes obstructive symptoms such as dyspnea, dysphagia, and hoarseness due to airway compression. It can also present with hypothyroidism, hypoparathyroidism or Horner’s syndrome.
Diagnosis can be made clinically with physical exam however a biopsy showing dense fibrous tissue with a characteristic eosinophilic infiltrate is needed for confirmation. Management includes both medical therapy and surgery for symptomatic relief. The mainstay of treatment is with glucocorticoids which can cause dramatic improvement of the symptoms. Alternatives include tamoxifen, mycophenolate, and radiation in situations where the patient is non-responsive to steroids.  Surgery is indicated when all other alternatives have failed, or when the patient has symptoms of airway compression.
The etiology of Riedel thyroiditis is not definitively known. Various theories have suggested that it could be a variant of autoimmune thyroiditis or part of a systemic fibrosing disorder or an immunoglobulin G4 (IgG4) related disease. The presence of anti-thyroid antibodies, eosinophilic infiltrate, and response to glucocorticoid therapy are suggestive of autoimmune pathology. However, the lack of other antibodies, association with other autoimmune disorders, normal lymphocyte counts and complement levels suggest otherwise. Riedel thyroiditis has been known to be associated with orbital fibrosis, primary sclerosing cholangitis, mediastinal and retroperitoneal fibrosis, giving rise to the suspicion that it may be a part of a multifocal idiopathic fibrosclerosing disorder. However, the lack of a consistent association of Riedel thyroiditis with other fibrosing disorders does not support this cause.
Out of the disorders that cause thyroiditis, Riedel's thyroiditis is extremely rare compared to Hashimoto's thyroiditis or subacute Granulomatous thyroiditis. The estimated incidence is suggested to be 1.06 cases per 100,000 people.  Patients are more likely to be adult females from 30-50 years of age. 
The hallmark of Riedel thyroiditis is the replacement of thyroid tissue with dense fibrotic tissue. The fibrosis involves extra-thyroidal structures including the trachea, parathyroid glands, neck musculature, laryngeal nerves, and blood vessels. This causes the thyroid to become immobile and fixed and is described as stone-hard or woody on palpation. 
Histologically, a dense hyalinized matrix with scanty colloid and a characteristic eosinophilic cell infiltrate can be seen. No malignant or giant cells are present.
The most common clinical presentation of Riedel thyroiditis is a hard and enlarged thyroid. Obstructive symptoms due to the involvement of neck structures are frequent. Dyspnea due to tracheal involvement, dysphagia due to esophageal involvement, stridor due to recurrent laryngeal nerve involvement, and venous sinus thrombosis due to the involvement of the vasculature can be present. Exophthalmos is secondary to the involvement of the retrobulbar tissues.
On examination, a hard mass is palpable in the anterior neck which may be immobile when swallowing as it is adherent to the surrounding neck structures. A positive Chvostek or Trousseau sign is indicative of hypoparathyroidism secondary to parathyroid gland involvement.
Thyroid function tests may reveal hypothyroidism in about 74% patients. Hashimoto thyroiditis can occur concomitantly, and thyroid peroxidase (TPO) antibodies are positive in about 90% of patients with Riedel thyroiditis. In rare instances, it can accompany Graves disease or subacute thyroiditis.
Ultrasonography (USG) of the neck would show a hypoechoic hypovascular mass involving the extra-thyroidal tissues and occasionally encasing the carotid vessels. Elastography would reveal stiff inflammatory tissues consistent with fibrosis. On computed tomography (CT) scan the mass appears as hypodense and does not enhance on the administration of contrast. CT scan can assess the extra-thyroidal involvement more accurately than USG. Positron emission tomography (PET) scan shows intense uptake in areas of inflammation and particularly useful in diagnosing remote areas of fibrosis which may occur in association with Riedel thyroiditis. Fine needle aspiration (FNA) of the mass is often inadequate but may show spindle cells, and fragments of fibrotic tissue. A definitive diagnosis can be made only by performing an open biopsy.
Diagnostic criteria for Riedel thyroiditis are :
There is no standardized treatment for Riedel thyroiditis due to the lack of outcome studies due to the rarity of the disease.
Surgery with a subtotal or partial thyroidectomy is indicated only to relieve compressive symptoms. Due to the lack of tissue planes between the fibrotic and normal thyroid tissue, surgical extirpation of the fibrotic tissue is rendered difficult. Unfortunately, complication rates as high as 39% have been reported even with limited surgical interventions. The consensus is to use minimal surgical intervention for the relief of compressive symptoms.
Glucocorticoids are the mainstay of medical treatment. The anti-inflammatory effects of glucocorticoids are most effective when used early in the disease process. There is no dosing guideline available however prednisone 15 mg to 100 mg daily has been shown to be effective in various reports. Response to the treatment is variable, with some cases showing dramatic improvement with a reversal of dysphonia and upper airway symptoms while others showed a complete lack of response. Nonresponse to glucocorticoids could be due to the increase in fibrosis seen in late disease as compared with early disease where inflammation is prominent.
Tamoxifen is a selective estrogen receptor modulator (SERM) used in the treatment of Riedel thyroiditis and other systemic fibrosing disorders. It induces tumor growth factor-beta (TGF-ß) which is a potent growth inhibitor. A dose of 10-20 mg, given alone or in combination with prednisone has been successful in decreasing the mass size.
Mycophenolate mofetil is an immunosuppressive agent with anti-fibrotic properties which has therapeutic use in systemic fibrosis. It converts to mycophenolic acid which inhibits the antibody production from T and B lymphocytes. Levy et al. successfully used a combination of mycophenolate and prednisone, in a case of Riedel thyroiditis not responsive to tamoxifen and prednisone however further studies are needed to assess the role of mycophenolate in the treatment of Riedel thyroiditis.
Riedel thyroiditis must be differentiated from other masses in the anterior neck which can infiltrate the surrounding extra-thyroidal tissues namely anaplastic thyroid carcinoma, thyroid lymphoma, and thyroid sarcoma.
Anaplastic carcinoma is an undifferentiated malignant tumor that is usually seen in older individuals that can clinically mimic Reidel thyroiditis with dysphagia or tracheal compression symptoms. Key differences are those with anaplastic carcinoma are older at 70 to 80 years of age, while those with Reidel thyroiditis are younger women in their fifth decade of life, and biopsy or FNA show no malignant cells.
Other differentials include a fibrosing variant of Hashimoto thyroiditis which is characterized by high titers of anti-thyroid antibodies, the presence of Hurthle cells, lack of extra-thyroidal extension and phlebitis.
This condition usually has a good prognosis. Mild disease can be stable for years, but can also be aggressive and rapidly progressive after any insult.
In general, there is a delay in diagnosis of up to 2 years due to the rarity and insidious nature of the disease. Mortality is for the most part due to tracheal compression. The disease-specific mortality rate has been reported to be around 6-10% in older studies. However, a recent study from Mayo clinic reported no increase in mortality over a follow-up period of 9.5 years and approximately 86% of patients had stable disease.
The complications from Riedel thyroiditis are secondary to involvement of the extra-thyroidal tissues by the fibrotic process. Tracheal compression can result in dyspnea, stridor, and respiratory failure. Occlusion of the neck vessels can result in venous thrombosis. Involvement of the sympathetic trunk can result in Horner’s syndrome. About 14% have involvement of the parathyroid glands which results in hypoparathyroidism.
Riedel thyroiditis has also been observed to occur in association with other fibrotic disorders. Mediastinal fibrosis can present as superior vena caval (SVC) syndrome due to occlusion of the SVC. Retroperitoneal fibrosis can present with back or flank pain secondary to hydroureteronephrosis. Abdominal pain due to sclerosing cholangitis or pancreatic fibrosis may also be present. Exophthalmos due to the involvement of retro-orbital soft tissues can also be a presenting feature.
Reidel thyroiditis is a rare disorder that often presents with vague signs and symptoms. Due to its rarity, it can be slow to diagnose (Level 4, 5). Patient care can be enhanced by a team of physicians, nurses, pharmacists and other health professionals. Once diagnosed, the patient will need routine followup with either a primary care physician or an endocrinologist for routine laboratory studies, imaging, and physical exams to monitor for progression of the disease. If the patient develops symptoms of tracheal compression despite medical therapy, they will need to be referred to a surgeon for a partial or subtotal thyroidectomy.
|||Hennessey JV, Clinical review: Riedel's thyroiditis: a clinical review. The Journal of clinical endocrinology and metabolism. 2011 Oct [PubMed PMID: 21832114]|
|||Zimmermann-Belsing T,Feldt-Rasmussen U, Riedel's thyroiditis: an autoimmune or primary fibrotic disease? Journal of internal medicine. 1994 Mar [PubMed PMID: 8120524]|
|||Heufelder AE,Hay ID, Evidence for autoimmune mechanisms in the evolution of invasive fibrous thyroiditis (Riedel's struma). The Clinical investigator. 1994 Oct [PubMed PMID: 7865983]|
|||Li Y,Bai Y,Liu Z,Ozaki T,Taniguchi E,Mori I,Nagayama K,Nakamura H,Kakudo K, Immunohistochemistry of IgG4 can help subclassify Hashimoto's autoimmune thyroiditis. Pathology international. 2009 Sep [PubMed PMID: 19712131]|
|||Katsikas D,Shorthouse AJ,Taylor S, Riedel's thyroiditis. The British journal of surgery. 1976 Dec [PubMed PMID: 1009341]|
|||Hay ID, Thyroiditis: a clinical update. Mayo Clinic proceedings. 1985 Dec [PubMed PMID: 3906289]|
|||Singer PA, Thyroiditis. Acute, subacute, and chronic. The Medical clinics of North America. 1991 Jan [PubMed PMID: 1987447]|
|||Heufelder AE,Goellner JR,Bahn RS,Gleich GJ,Hay ID, Tissue eosinophilia and eosinophil degranulation in Riedel's invasive fibrous thyroiditis. The Journal of clinical endocrinology and metabolism. 1996 Mar [PubMed PMID: 8772560]|
|||Fatourechi MM,Hay ID,McIver B,Sebo TJ,Fatourechi V, Invasive fibrous thyroiditis (Riedel thyroiditis): the Mayo Clinic experience, 1976-2008. Thyroid : official journal of the American Thyroid Association. 2011 Jul [PubMed PMID: 21568724]|
|||Marín F,Araujo R,Páramo C,Lucas T,Salto L, Riedel's thyroiditis associated with hypothyroidism and hypoparathyroidism. Postgraduate medical journal. 1989 Jun [PubMed PMID: 2608578]|
|||Papi G,Corrado S,Cesinaro AM,Novelli L,Smerieri A,Carapezzi C, Riedel's thyroiditis: clinical, pathological and imaging features. International journal of clinical practice. 2002 Jan-Feb [PubMed PMID: 11831840]|
|||Ozgen A,Cila A, Riedel's thyroiditis in multifocal fibrosclerosis: CT and MR imaging findings. AJNR. American journal of neuroradiology. 2000 Feb [PubMed PMID: 10696016]|
|||Papi G,LiVolsi VA, Current concepts on Riedel thyroiditis. American journal of clinical pathology. 2004 Jun [PubMed PMID: 15298150]|
|||Vaidya B,Harris PE,Barrett P,Kendall-Taylor P, Corticosteroid therapy in Riedel's thyroiditis. Postgraduate medical journal. 1997 Dec [PubMed PMID: 9497955]|
|||Bagnasco M,Passalacqua G,Pronzato C,Albano M,Torre G,Scordamaglia A, Fibrous invasive (Riedel's) thyroiditis with critical response to steroid treatment. Journal of endocrinological investigation. 1995 Apr [PubMed PMID: 7560814]|
|||Thomson JA,Jackson IM,Duguid WP, The effect of steroid therapy on Riedel's thyroiditis. Scottish medical journal. 1968 Jan [PubMed PMID: 5694137]|
|||Few J,Thompson NW,Angelos P,Simeone D,Giordano T,Reeve T, Riedel's thyroiditis: treatment with tamoxifen. Surgery. 1996 Dec [PubMed PMID: 8957485]|
|||Levy JM,Hasney CP,Friedlander PL,Kandil E,Occhipinti EA,Kahn MJ, Combined mycophenolate mofetil and prednisone therapy in tamoxifen- and prednisone-resistant Reidel's thyroiditis. Thyroid : official journal of the American Thyroid Association. 2010 Jan [PubMed PMID: 20067381]|
|||Torres-Montaner A,Beltrán M,Romero de la Osa A,Oliva H, Sarcoma of the thyroid region mimicking Riedel's thyroiditis. Journal of clinical pathology. 2001 Jul [PubMed PMID: 11429435]|
|||Wan SK,Chan JK,Tang SK, Paucicellular variant of anaplastic thyroid carcinoma. A mimic of Reidel's thyroiditis. American journal of clinical pathology. 1996 Apr [PubMed PMID: 8604680]|
|||Vigouroux C,Escourolle H,Mosnier-Pudar H,Thomopoulos P,Louvel A,Chapuis Y,Varet B,Luton JP, [Riedel's thyroiditis and lymphoma. Diagnostic difficulties]. Presse medicale (Paris, France : 1983). 1996 Jan 6-13 [PubMed PMID: 8728889]|