Description
Riedel thyroiditis, also known as Riedel’s struma or invasive fibrous thyroiditis, is a rare chronic inflammatory condition of the thyroid gland characterised by the replacement of normal thyroid parenchyma by fibrous tissue that invades adjacent structures in the neck. This condition is part of the spectrum of IgG4-related systemic diseases, although it can exist independently of other systemic involvement.
Pathogenesis
The pathogenesis of Riedel thyroiditis remains unclear, but it is considered an IgG4-related systemic disease. There is an increased number of IgG4-positive plasma cells in affected tissues, suggesting an autoimmune etiology. However, the triggers and exact mechanisms are yet to be fully elucidated.
Epidemiology, Risk Factors & Associations
- Riedel thyroiditis is a very rare disorder, with fewer than one in 100,000 people affected per year (0.06/100,000 per year).
- It is more common in middle-aged women.
- Associated with other fibrotic disorders such as retroperitoneal fibrosis, fibrous mediastinitis, orbital pseudotumor, and sclerosing cholangitis but the cause of these associations is unknown.
Clinical Features
- The primary clinical feature of Riedel thyroiditis is a hard, fixed, painless goitre that can lead to compressive symptoms such as dysphagia, dyspnea, and hoarseness due to tracheal or oesophageal compression.
- It is often misdiagnosed as anaplastic thyroid carcinoma due to its clinical features.
Complications
Due to the fibrotic nature of the condition, complications can arise from compression of adjacent structures such as the trachea, oesophagus, and recurrent laryngeal nerves. This can lead to dysphagia, dyspnea, and hoarseness.
Pathological Features
Histopathology
Histopathological examination reveals dense fibrosis that extends beyond the thyroid capsule into adjacent structures. This fibrosis can obliterate the normal thyroid architecture and can be accompanied by a sparse lymphocytic infiltrate.
Serology
There are no specific serological markers for Riedel thyroiditis. However, in some cases, an elevated sedimentation rate, mild leukocytosis, and/or hypoalbuminemia may be present.
Biochemistry
- Elevated ESR and CRP
- Elevated serum levels of thyroglobulin.
- Normal to mildly elevated thyroid-stimulating hormone (TSH).
- Normal to low free thyroxine (T4) levels.
- Negative thyroid autoantibodies (thyroid peroxidase and thyroglobulin antibodies)
Radiological Features
General Features
On imaging, the thyroid gland is typically enlarged and infiltrative, with a decrease in the size and number of nodules.
US
- The thyroid gland is diffusely enlarged with heterogeneous echotexture.
CT
- The thyroid gland shows diffuse enlargement with low attenuation.
MRI
- T1: The gland is typically hypointense.
- T2: The gland shows low signal intensity due to dense fibrosis.
Diagnosis
The diagnosis is generally made on the basis of clinical presentation, imaging, and histopathology, which shows dense fibrous tissue replacing normal thyroid parenchyma.
Differential Diagnosis
- Anaplastic thyroid carcinoma: This aggressive thyroid cancer may present similarly with a rapidly enlarging mass, however, it often presents with a more aggressive course and a worse prognosis. On imaging, anaplastic thyroid carcinoma often shows more destructive changes and invades local structures more extensively than Riedel thyroiditis. Histologically, anaplastic thyroid carcinoma shows undifferentiated cells, which is not a feature of Riedel thyroiditis.
- Hashimoto’s thyroiditis: Hashimoto’s thyroiditis can also cause thyroid enlargement and can sometimes present with a firm goitre. However, patients with Hashimoto’s often have hypothyroid symptoms and the presence of anti-thyroid antibodies (anti-TPO, anti-TG) in the serum.
- Thyroid lymphoma: This is a rare cause of a rapidly enlarging thyroid mass. It typically presents in older individuals and may be associated with Hashimoto’s thyroiditis. On imaging, thyroid lymphoma often shows a more homogeneous enlargement of the thyroid without the infiltrative pattern seen in Riedel thyroiditis. Diagnosis is confirmed by biopsy showing malignant lymphoid cells.
- Medullary thyroid carcinoma (MTC): MTC is a neuroendocrine tumour that arises from the parafollicular C cells of the thyroid. It often presents as a firm thyroid mass and may produce calcitonin, leading to diarrhoea. On imaging, MTC tends to be solitary and can calcify, unlike the diffuse pattern in Riedel thyroiditis.
- Subacute (De Quervain’s) thyroiditis: This is a viral-induced inflammation of the thyroid that can present with a painful, enlarged thyroid. However, the thyroid is often tender in De Quervain’s, in contrast to the painless enlargement seen in Riedel thyroiditis. Laboratory findings often show elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP).
Management
The management of Riedel thyroiditis is primarily medical, with surgery reserved for cases with severe compressive symptoms. Medical treatment usually includes corticosteroids to reduce inflammation and fibrosis. In refractory cases, tamoxifen or rituximab may be used.