Background: Hypothyroidism is a common endocrine disorder that can manifest with multisystem involvement.

Neuropsychiatric manifestations are well recognized; however, frank psychosis due to hypothyroidism, known as myxedema psychosis, is rare. Because psychiatric symptoms may dominate the clinical presentation, the underlying endocrine disorder may remain undiagnosed for prolonged periods.

Case Presentation: We report the case of a 35-year-old female who presented with suspicious behaviour for nearly 6–7 years, with significant worsening over the previous 4–5 months. She developed auditory hallucinations, persecutory and infidelity delusions, low mood, and progressive social withdrawal. There was no history of substance abuse, seizures, or neurological illness. Physical examination revealed pallor with stable vital signs. Mental status examination showed psychomotor retardation, reduced speech output, dysphoric affect, and second-person auditory hallucinations with preserved orientation. Laboratory investigations revealed markedly elevated thyroid-stimulating hormone (>100 µIU/mL) with low serum T3 and T4 levels. Anti-thyroid peroxidase antibodies were markedly elevated, and ultrasonography of the neck demonstrated diffuse thyroiditis, suggestive of Hashimoto’s thyroiditis. Hemoglobin was 8.3 g/dL with low ferritin levels consistent with iron deficiency anemia. Other routine investigations were within normal limits.

Management and Outcome: A diagnosis of psychotic disorder secondary to severe autoimmune hypothyroidism was established. The patient was treated with levothyroxine replacement therapy along with risperidone for psychotic symptoms and oral iron supplementation. At three-month follow-up, there was marked clinical improvement with resolution of hallucinations and significant improvement in mood, insight, and daily functioning. Improvement in psychiatric symptoms paralleled normalization of thyroid function tests. Conclusion: