Riedel’s thyroiditis is considered to have autoimmune etiology and may be caused by the eosinophilia and the proliferation fibroblast as a result of cytokines released by inflammatory cells. Although some drugs such as methysergide, serotonin, lysergic acid, and ergotamine have also been identified as the cause of retroperitoneal fibrosis, there are no reports of the direct association with Riedel’s thyroiditis.

Causes of Riedel’s thyroiditis may include:^{[1]_The_Journal_of_Clinical_Endocrinology_&_Metabolism_|_Oxford_Academic-2|[2][3][4]}

• Autoimmunity
  • T and B Lymphocyte and macrophage infiltration
  • Eosinophilic infiltration

• Drugs
  • There are no reports of the direct association with Riedel’s thyroiditis with drugs. Some drugs have been identified as the cause of retroperitoneal fibrosis. As Riedels’ thyroiditis can develop as a manifestation of multi organ fibrosis these drugs are still relevant even though the direct link is not established. Some of the drugs which can cause retroperitoneal fibrosis are:
    • Methysergide
    • Serotonin
    • Lysergic acid
    • Ergotamine
    • Dihydroergotamine
    • Hydralazine
    • Methyldopa
    • Acetazolamide
    • Phenacetin
    • Beta-blockers

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