Disappearance of Posterior Pituitary Hyperintensity in Arginine Vasopressin Resistance: A Case Report.

The evaluation of arginine vasopressin resistance (AVP-R) and arginine vasopressin deficiency (AVP-D), respectively, previously referred to as nephrogenic and central diabetes insipidus, often includes brain magnetic resonance imaging (MRI). Its purpose is to assess pituitary structural changes and identify the physiological T1 hyperintensity of the posterior pituitary, present in most individuals. This hyperintensity reflects vasopressin storage, which shortens the T1 relaxation time. Its absence supports a diagnosis of AVP-D over AVP-R. A 13-year-old boy was referred to endocrinology in his second year of life for polyuria and polydipsia. Brain MRI revealed normal pituitary morphology and a spontaneous T1 hyperintensity of the posterior pituitary. The AVP-R was confirmed by a hemizygous AVPR2 gene mutation. Treatment with hydrochlorothiazide, potassium supplementation, and dietary sodium restriction achieved good clinical control. During school age, progressive growth deceleration led to short stature and delayed puberty. At 13 years, repeated MRI showed a smaller pituitary gland and loss of the posterior pituitary T1 hyperintensity. Loss of posterior pituitary T1 hyperintensity is rare in AVP-R, and late disappearance has not been previously described in genetically confirmed cases. This case underscores the importance of integrating clinical, genetic, and imaging findings.