BMJ Case Reports 2015; doi:10.1136/bcr-2013-009111

Reset osmostat: a rare cause of hyponatraemia

  1. Gabriela Mimoso2
  1. 1Hospital Pediátrico, Centro Hospitalar Universitário de Coimbra, Portugal
  2. 2Maternidade Bissaya Barreto do Centro Hospitalar Universitário de Coimbra, Coimbra, Portugal
  1. Correspondence to Dr Beatriz Vale, beatriz_vale{at}
  • Accepted 3 June 2015
  • Published 29 June 2015


Neonatal hyponatraemia is common, and related to significant morbidity and mortality. We report a case of a preterm newborn (gestational age of 36 weeks) with hyponatraemia, and with a prenatal diagnosis of cleft lip and palate, with a normal fetal karyotype. On the seventh day of life, a biochemical evaluation for jaundice and mild signs of dehydration showed hyponatraemia of 124 mmol/L. Investigation showed normal adrenal and thyroid functions, plasma hyposmolality (258 mOsm/kg); high urinary sodium (73 mmol/L) and high urinary osmolality (165 mOsm/kg). Despite oral sodium supplementation and fludrocortisone treatment, sodium levels remained between 124 and 130 mmol/L. Cranial ultrasound, brain MRI and renal ultrasound were normal. The diagnosis of hyponatraemia was unpredicted and the investigation was suggestive of reset osmostat, a subtype of the syndrome of inappropriate secretion of antidiuretic hormone, characterised by a subnormal threshold for antidiuretic hormone secretion.

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