Why Is TSH Usually Normal in Hashimoto's Encephalopathy?

Despite its name, Hashimoto’s encephalopathy—also referred to in medical literature as SREAT (Steroid-Responsive Encephalopathy Associated with Autoimmune Thyroiditis)—is not caused by a lack of thyroid hormones. Instead, it is a rare neurological syndrome where the immune system misfires. Because the disease process is linked to brain inflammation rather than a failure of the thyroid to produce hormones, many patients have normal thyroid function (a euthyroid state) when they are diagnosed ^[1][2].

Understanding the Misleading Name

The term “Hashimoto’s” usually brings to mind Hashimoto’s thyroiditis, a common condition that slowly damages the thyroid and leads to an underactive thyroid (hypothyroidism). However, in Hashimoto’s encephalopathy, the link is not the hormone levels themselves, but the presence of anti-thyroid antibodies, such as anti-TPO (thyroid peroxidase) or anti-Tg (thyroglobulin) ^[3].

These antibodies indicate that the immune system is targeting thyroid-related proteins. However, these antibodies are very common in the general population, and their presence alone does not prove a person has SREAT. Medical literature reviews show that patients with this encephalopathy can have normal, slightly low, or high thyroid hormone levels ^[1][4]. In a systematic review of over 250 reported cases, the majority of patients had a normal TSH (Thyroid Stimulating Hormone, a pituitary hormone that signals the thyroid to produce the hormone T4) ^[1].

Furthermore, the severity of a patient’s neurological symptoms does not match their thyroid hormone levels or the concentration of these antibodies in their blood ^[5][6]. Because anti-thyroid antibodies are so nonspecific, SREAT is generally a “diagnosis of exclusion,” meaning doctors must first rule out other causes of brain dysfunction, infections, structural issues, or metabolic problems before confirming it ^[7][3].

How Does the Immune System Affect the Brain?

If thyroid hormone production is normal, why is the brain affected? Scientists are still studying this, and no single mechanism has been proven. Current evidence suggests that anti-thyroid antibodies may not actually cause the brain damage directly, but rather act as “bystanders” that hint at a broader immune issue ^[7]. Researchers have proposed several unproven hypotheses for how this might happen:

• Shared Targets (Cross-reactivity): The immune system might produce antibodies that mistakenly target proteins in the brain because they look structurally similar to thyroid proteins ^[8].
• Blood-Brain Barrier Inflammation: The blood-brain barrier is a protective shield between your bloodstream and your brain tissue. Some researchers suspect the condition involves inflammation in the brain’s blood vessels, allowing immune cells to inappropriately cross this barrier ^[9][10].
• Unidentified Brain-Specific Antibodies: The immune system may simultaneously be producing other, yet-to-be-identified antibodies that directly attack the surface of brain cells, while the anti-thyroid antibodies are merely present at the same time ^[11][3].

What Normal Thyroid Labs Mean for Your Care

Having normal thyroid lab results (like TSH and free T4) means that taking synthetic thyroid hormone replacement will not fix your neurological symptoms, because your body is already naturally producing the right amount of hormone. Since SREAT is fundamentally an immune system disorder targeting the central nervous system, treatment focuses on calming the immune response, typically with corticosteroids or other immunosuppressants (medications that lower immune activity).

Important Safety and Treatment Context:
Because immunosuppressants carry significant risks—such as increased vulnerability to infection, blood sugar spikes, blood pressure changes, and mood shifts—they must be carefully prescribed and managed by a specialist. Do not start, stop, or change the dose of steroids without your doctor’s supervision.

Additionally, because this condition involves the brain, seek urgent or emergency medical care if you or your loved one experiences a first or prolonged seizure, an inability to stay awake, rapidly worsening confusion, new one-sided weakness, or severe behavioral changes that pose an immediate danger. Routine thyroid monitoring is not a substitute for emergency neurological care.