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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\u2009mmol\/L), severe CH (TSH 300 mIU\/L, free T4 2\u2009pmol\/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.<\/jats:p>","DOI":"10.1136\/bcr-2024-262474","type":"journal-article","created":{"date-parts":[[2025,1,31]],"date-time":"2025-01-31T12:05:46Z","timestamp":1738325146000},"page":"e262474","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":1,"title":["Hypothyroid-induced hyponatraemia in a premature infant with a false-negative newborn screening for congenital hypothyroidism"],"prefix":"10.1136","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0009-0006-4083-3519","authenticated-orcid":false,"given":"Maartje","family":"van Gisbergen","sequence":"first","affiliation":[{"name":"Pediatrics, Elisabeth-TweeSteden Ziekenhuis, Tilburg, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0002-7015-6388","authenticated-orcid":false,"given":"Ronald","family":"de Moor","sequence":"additional","affiliation":[{"name":"Pediatrics, Elisabeth-TweeSteden Ziekenhuis, Tilburg, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3999-8854","authenticated-orcid":false,"given":"Annemarie","family":"Verrijn Stuart","sequence":"additional","affiliation":[{"name":"Wilhelmina Children's Hospital University Medical Centre, Utrecht, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0002-6396-6585","authenticated-orcid":false,"given":"Gerrit","family":"van den Berg","sequence":"additional","affiliation":[{"name":"Wilhelmina Children's Hospital University Medical Centre, Utrecht, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-4932-6348","authenticated-orcid":false,"given":"Bianco","family":"Sijbesma","sequence":"additional","affiliation":[{"name":"Wilhelmina Children's Hospital University Medical Centre, Utrecht, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0008-9426-2576","authenticated-orcid":false,"given":"Jacorina","family":"van Hoften","sequence":"additional","affiliation":[{"name":"Pediatrics, Elisabeth-TweeSteden Ziekenhuis, Tilburg, The Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"239","published-online":{"date-parts":[[2025,1,31]]},"reference":[{"key":"2025082709110458000_18.1.e262474.1","doi-asserted-by":"crossref","first-page":"175","DOI":"10.1016\/j.beem.2013.05.008","article-title":"Screening for congenital hypothyroidism: a worldwide view of strategies","volume":"28","author":"Ford","year":"2014","journal-title":"Best Pract Res Clin Endocrinol Metab"},{"key":"2025082709110458000_18.1.e262474.2","unstructured":"Gezondheidsraad . 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