Change of the image of the posterior pituitary in a patient with mineralocorticoid-responsive hyponatremia of the elderly: comparison of findings before and after treatment.

On October 5, 2005, an 80-year-old woman with no contributory past history consulted our hospital with a complaint of headache. Laboratory studies showed serum sodium of 119 mEq/l and potassium of 4.6 mEq/l. Urinary excretion of sodium was 61 mEq/l. Blood urea nitrogen was 11 mg/dl;serum creatinine, 0.5 mg/dl;serum uric acid, 2.0 mg/dl;and hematocrit, 38.0%. The plasma osmolality was low at 247 mOsm/kgH2O, while urinary osmolality was somewhat elevated at a concentration of 333 mOsm/kgH2O. The thyroid stimulating hormone (TSH) was 1.87 μIU/ml and free thyroxine, 1.3 ng/dl. Adrenocorticotropin (ACTH) was 55.0 pg/ml, cortisol, 22.7 μg/dl and arginine vasopressin (AVP) was relatively high at 0.4 pg/ml compared to the low plasma osmolality. The patient was diagnosed as having a syndrome of inappropriate secretion of antidiuretic hormone (SIADH). She had not taken any medication that would induce SIADH. Computed tomographic (CT) scan of the whole body did not demonstrate pleural effusion, ascites or findings of malignancy. On cranial magnetic resonance imaging (MRI), there were no abnormal findings except for the absence of high intensity of the posterior pituitary on T1weighted images (Fig. 1A). Despite restriction of water intake, the serum sodium level decreased to 116 mEq/l, and amnesia appeared. Therefore, administration of fludrocortisone acetate 0.05 mg was initiated on suspicion of mineralocorticoid-responsive hyponatremia of the elderly (MRHE). The serum sodium level improved to 122 mEq/l on October 18 and 134 mEq/l on October 26. With improvement of the serum sodium level, her headache disappeared and amnesia improved. Serum sodium was 139 mEq/l; serum uric acid, 4.6 mg/dl; and hematocrit, 38.9% on December 14, then administration of fludrocortisone acetate was discontinued. Laboratory data showed a serum sodium level of 135 mEq/l on December 21; plasma osmo-