This case report highlights the infrequent occurrences of myxedema psychosis and pituitary hyperplasia as secondary complications of Hashimoto’s thyroiditis. The subject of interest is a 16-year-old female patient who displayed persistent symptoms of delusions, auditory hallucinations, and depression without any discernible cause. The physical examination did not reveal any notable abnormalities. Despite a continuous 4-week treatment regimen involving antipsychotic and antidepressant medications, the patient experienced a deterioration of psychotic symptoms, which resulted in substantial impairment in social functioning. Cranial magnetic resonance imaging scan detected an enlarged pituitary gland and laboratory tests indicated abnormal thyroid function and the presence of specific thyroiditis autoantibodies. Combined with these results, thyroid ultrasound findings revealed features that were consistent with Hashimoto’s thyroiditis. Consequently, the patient was prescribed levothyroxine sodium replacement therapy, as well as low-dose antipsychotic and antidepressant medications, which led to a gradual amelioration of their psychotic symptoms. Following 6 months of treatment, the patient was ordered to stop taking the antipsychotic and antidepressant drugs but continue with levothyroxine sodium replacement therapy exclusively. The results of the 1-year follow-up demonstrated the restoration of thyroid function to normal levels, the normalization of pituitary size and structure, and the absence of any psychotic symptoms. These findings provide evidence for a diagnosis of myxedema psychosis and secondary pituitary hyperplasia caused by hypothyroidism associated with Hashimoto’s thyroiditis. This emphasizes the significance of supplementary tests in the diagnostic procedures carried out by psychiatrists.