We describe a case of myotonic dystrophy presenting with a disturbed circadian rhythm of the serum cortisol and an isolated thyrotropin deficiency. The diagnosis of myotonic dystrophy was based on clinical characteristics, positive electromyographic findings, and increased number of CTG repeats in the dystrophia myotonica protein kinase (DMPK) gene. The patient presented with a variable circadian rhythm of the serum cortisol, increased excretion of urinary free cortisol, and a high adrenocorticotropin hormone responses to corticotropin-releasing hormone. The basal serum thyrotropin concentration was low and did not increase after thyrotropin-releasing hormone stimulation. The protein encoded by the DMPK gene may act as a second messenger in signal transduction, like a protein kinase. The present patient had a diverse pattern of disturbances in the hypothalamus-pituitary-endocrine organ axis, probably mediated by differences in the action or expression of the gene products in each endocrine cell.