Hashimoto thyroiditis was discovered by Hakaru Hashimoto in 1912 but became more recognized in the 1950s. It is an autoimmune disorder with an incidence rate of about 0.8/1000/year in men and 3.5/1000/year in women. The inheritance pattern of Hashimoto thyroiditis is not fully understood and diagnosing this condition could be challenging. Among many presentations, its effects on mental health can lead to a greater burden on a patient. There has been an increased report of acute psychiatric symptoms in this condition. Literature has described a wide spectrum of psychiatric manifestations occurring prior to, during, and after this illness. The aim of this report is to describe a woman with diagnosed psychosis secondary to Hashimoto's thyroiditis.

Ms S is a 22-year-old female who was admitted in January 2021 to the emergency department of Hospital B with a history of sudden behavioural changes: agitation, responding to unseen stimuli, and bizarre behaviour.

Her previous record reveals that she had a similar presentation in November 2019, managed with antipsychotics. The diagnosis at the time was unclear however; meningoencephalitis was suspected and later for NDMA encephalitis. After 4 months of admission to Hospital A, her behavioural changes remained unresolved but she was discharged to a care home with 2:1 support. She remained in the care home until further deterioration, which warranted a further admission to Hospital B.

Following a psychiatric review in Hospital B, she was initially diagnosed with an Acute psychotic episode with query cause and managed with IM Aripiprazole.

Due to physical health concerns and the unclear nature of her diagnosis, she was transferred to the acute medical ward and further investigation was requested.

Her result showed significantly elevated Thyroid Peroxidase Antibodies of 845 IU/ml (normal up to 24 IU/ml), lumbar puncture and NMDA antibody test were both normal, TSH level was raised to 6.73. Following further discussion with the medical team, a diagnosis of Psychosis secondary to Hashimoto's thyroiditis was made.

She was co-managed by the psychiatrist, endocrinologist and others.

Ms S became settled but due to residual psychosis, she was transferred to an inpatient psychiatric ward where her psychosis resolved, and was discharged back to the community mental health services.

As a result of the late diagnosis of her condition and delayed hospital stay, Ms. S developed a lot of mistrust for the services as she believe that the 19-month delay had significantly impacted her quality of life.

Diagnosis of psychosis secondary to Hashimoto's thyroiditis requires a high index of suspicion, missing this could lead to inappropriate use of medication and increased mobility.