(Disclaimer: This article is for scientific purposes only. To protect patient privacy, the relevant information in the following content has been processed) Abstract: In clinical practice, some patients with dementia may be caused by autoimmune thyroid disease, which requires differential diagnosis with other encephalopathies. In the present case, the patient presented with memory and calculation loss without any obvious reason in the past 1 year, accompanied by slow reaction time, with progressive aggravation, and was considered to have secondary dementia due to thyroiditis after consultation. After medication, the patient’s symptoms improved, his condition stabilized, and he was successfully discharged from the hospital. Basic information] Male, 55 years old [Disease type] Dementia, Hashimoto’s encephalopathy, hypothyroidism [Hospital] The Third Hospital of Shandong Province [Time of consultation] May 2020 [Treatment plan] Medication (levothyroxine sodium tablets, prednisone acetate tablets) [Treatment cycle] Hospitalization for 7 days, outpatient follow-up after 1 month [Treatment effect] Stability of the condition and improvement of the symptoms I. Initial consultation The patient came to our hospital and his symptoms improved. A patient came to our hospital. Although the patient was not too old, he was very slow in communicating with others and needed to think for a long time. After communication, we learned that the patient had suffered from Hashimoto’s thyroiditis for many years, and his condition had always been relatively stable, but in the past year, his family found that the patient’s forgetfulness had gradually worsened, such as forgetting what he had eaten in the morning, forgetting to bring his keys and cell phone when he went out, failing to find his clothes, and not being able to count the money when he bought groceries, etc., and found that he was obviously slow in communicating with the patient. At first, the family thought that the above symptoms were age-related and normal, but later found that the symptoms were progressively worsening, so he came to the hospital. A cognitive function assessment test was performed, suggesting mild-moderate cognitive dysfunction, and a preliminary diagnosis of dementia and hypothyroidism was made at the outpatient clinic. For further treatment, he was admitted to the hospital. We communicated with the patient’s family and indicated that cranial magnetic resonance examination, blood test, liver and kidney function test, blood lipid test, blood glucose test, thyroid function test, etc. were needed to be improved in the follow-up, and the family expressed their understanding and actively cooperated with us. After the patient was admitted to the hospital, blood tests were completed, including routine blood tests, liver and kidney function tests, blood lipids, blood glucose, cardiac enzymes, glycated hemoglobin, thyroid function tests, etc. At the same time, cerebrospinal fluid lumbar puncture test was completed, and cranial magnetic resonance test (cranial MR), cranial magnetic resonance angiography, electroencephalogram, etc. The results suggested that the patient’s thyroid gland was not in a good condition, and the patient had to undergo cerebral magnetic resonance test. The examination results suggested that the patient’s thyroid function was hypothyroid and antibodies were elevated; cerebrospinal fluid laboratory results suggested that the patient’s protein was mildly elevated; craniocerebral magnetic resonance suggested multiple lacunar infarct foci and ischemic foci, and other obvious abnormalities were not seen. Combined with the patient’s medical history, the diagnosis considered dementia, Hashimoto’s encephalopathy, and hypothyroidism. After admission, high-dose hormone shock therapy was given, including levothyroxine sodium tablets and prednisone acetate tablets, while actively improving cognitive function and controlling hypothyroidism. After 1 week of medication, the patient’s cognitive function symptoms improved compared with the previous period, and his memory and calculation ability improved. He was discharged from the hospital after 1 week of treatment, and was instructed to take the medication regularly and to return to the outpatient clinic 1 week later. After the patient was admitted to the hospital, relevant examinations were actively completed, and combined with the patient’s past medical history and laboratory results, dementia, Hashimoto’s encephalopathy and hypothyroidism were comprehensively determined. Subsequently, the patient was actively treated with high-dose shock therapy with hormone drugs. 3 days after treatment, the patient’s cognitive symptoms improved, and after 1 week of treatment, the patient’s cognitive symptoms improved significantly and his condition stabilized, so he was allowed to be discharged from the hospital and instructed to go to the hospital for follow-up on a regular basis. One week after discharge, the patient went to the outpatient clinic for follow-up, complaining of no special discomfort. Afterwards, the patient’s thyroid function was rechecked, and the patient’s antibodies turned negative, but the thyrotropin was still high, so the drug dose was adjusted. 1 month later, the patient was followed up by telephone, and it was learned that the patient’s cognitive dysfunction had improved, and the patient was now in a stable state, and his condition had not worsened. After a series of treatments, the patient’s symptoms improved significantly, and his condition stabilized without exacerbation, which is the most desirable result for our clinicians. The patient still needs to pay attention to the following situations after discharge: 1. The patient should regularly monitor the thyroid function after discharge, and if signs of neurological dysfunction reappear within a short period of time, the patient should actively go to the hospital; 2. The patient should develop good dietary and sleeping habits after discharge, and at the same time, in the diet, the iodine intake needs to be controlled in order to avoid exacerbation of the condition. Patients also need to take appropriate selenium supplementation and eat more selenium-rich foods, such as meat and seafood, etc. Meanwhile, they should quit smoking and drinking, and avoid spicy, stimulating and greasy diet. Fifth, personal perception dementia can be seen in a variety of diseases, including dementia caused by thyroid disease is a rare disease, the onset of the disease and autoimmune thyroid disease related to the neurological symptoms can often be caused by intracerebral nervous system symptoms, the patient will often be manifested as dementia, epilepsy, cerebral infarction, etc., the need for differential diagnosis with other brain diseases. The patient in this article belongs to the dementia caused by thyroid disease, therefore, the symptoms are similar to those of intracerebral diseases. Therefore, clarifying the patient’s detailed medical history will help to diagnose the patient’s condition and lay a good foundation for early treatment.