Hashimoto encephalopathy is a controversial and poorly understood diagnosis classically defined as a steroid-responsive encephalopathy associated with thyroid autoimmunity. The key features are: (1) altered level of consciousness and (2) presence of anti-thyroid peroxidase (anti-TPO) or anti-thyroglobulin (anti-TG) antibodies. It is distinct from alterations in mentation that may occur due to severe hyper- or hypothyroidism, and severe abnormalities in thyroid hormone levels should be excluded when making the diagnosis. Symptoms can vary considerably though commonly include cognitive deficits, behavioral changes, myoclonus, stroke-like episodes, and seizures.
Patients with suspected Hashimoto encephalopathy require careful evaluation for autoimmune and nonautoimmune causes of encephalopathy.
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• Hashimoto encephalopathy is a poorly understood syndrome of reversible cognitive impairment in the setting of antithyroid autoantibodies.
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• There are no validated criteria for the diagnosis of the disease but proposed diagnostic criteria include: (1) encephalopathy with seizures, myoclonus, or stroke-like episodes; (2) subclinical or mild overt thyroid disease; (3) normal or nonspecific abnormalities on brain MRI; (4) positive serum anti-TPO or anti-TG antibodies; (5) absence of well-characterized neuronal antibodies in CSF and serum; (6) reasonable exclusion of alternative causes.
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• The literature on Hashimoto encephalopathy is highly heterogenous and most reports did not adequately exclude alternative diagnoses, such as other causes of autoimmune encephalitis.
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• Antithyroid antibodies are relatively common in the general population and may also be found in patients with other, better-defined types of autoimmune encephalitis.
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• The existence of the disease has been seriously questioned in recent years owing to its nonspecific diagnostic criteria and to a lack of distinct biomarkers.
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• There is no class 1 evidence for treatment of Hashimoto encephalopathy.
Historical note and terminology
In 1966, Brain and colleagues reported the initial case of Hashimoto encephalopathy, a 40-year-old man with antithyroid antibodies who had multiple stroke-like episodes and periods of profound confusion (03). He eventually had excellent recovery (although he did not obviously respond to corticosteroids). Though he had thyroid disease, this was well controlled during most of his clinical course. The authors concluded that he may have had an autoimmune disease but doubted that the antithyroid antibodies were directly pathogenic.
In the last decade, alternative names for Hashimoto encephalopathy have been proposed, including steroid-responsive encephalopathy associated with thyroiditis (SREAT) and encephalopathy associated with autoimmune thyroid disease (EAATD).
Hashimoto encephalopathy should not be confused with Hashimoto thyroiditis. Hashimoto thyroiditis was first described in 1912 as a form of thyroid disease with lymphomatous infiltrates (15). Unlike Hashimoto encephalopathy, humoral factors such as antibodies to thyroid peroxidase, thyroglobulin, and other antithyroid antibodies are thought to directly contribute to the pathogenesis of Hashimoto thyroiditis, along with cellular immunity, genetic risk, and other factors (01). The diagnosis of Hashimoto thyroiditis is based on the presence of primary hypothyroidism (elevated TSH, low T4), goitrous or atrophic thyroid changes, and can be confirmed by the presence of anti-TPO or anti-TG antibodies.