Hypothyroid-induced hyponatraemia in a premature infant with a false-negative newborn screening for congenital hypothyroidism

Worldwide newborn screening (NBS) programmes detect one in 2000-4000 newborns with forms of congenital hypothyroidism (CH). However, some newborns such as premature born babies, are at increased risk for a false-negative test result caused by delayed thyroid-stimulating hormone (TSH) elevation through an immature hypothalamic-pituitary axis. We report a case of a premature newborn in which CH remained undetected. At the age of 3 months cardiorespiratory incidents increased, bradycardia, hypothermia and seizures developed. Investigation revealed severe hypotonic hyponatraemia (85 mmol/L), severe CH (TSH 300 mIU/L, free T4 2 pmol/L), renal insufficiency and metabolic acidosis. High urine osmolality, low urine sodium concentration and an extremely activated renin-angiotensin-aldosterone system suggested a low effective circulating volume. This case report describes hypothyroid-induced hyponatraemia in a premature newborn with a false-negative NBS and thus re-emphasises the importance of repeat screening or repeat thyroid function testing for CH in premature newborns.