ESPE Abstracts

Background: Mild isolated neonatal hyperthyrotropinaemia (HTT), defined as elevated thyroid stimulating hormone (TSH) with normal free thyroxine (FT4) concentrations, may be identified by newborn screening programmes for congenital hypothyroidism (CHT) or when neonatal thyroid function tests (TFTs) are performed for a clinical indication. Reported HTT incidence rates vary (from 0.001-0.1%)¹. Case definition also varies, with some authors using a TSH threshold of up to 30 mU/L¹. HTT is usually transient and self-resolving, but permanent CHT requiring treatment has been reported². We aimed to ascertain the incidence of HTT in a large Irish cohort.

Methods: A 5 year retrospective, observational review was performed across three sites (National Newborn Screening Centre and two tertiary maternity hospitals). All infants less than one month of age with a serum TSH level above the normal reference range (5.5mU/L) but less than 10mU/L and a normal FT4 value (10-22pmol/L) were identified. Demographic, clinical, and serial biochemical data were reviewed.

Results: Of 64,272 infants born during the study period, 426 neonates were identified as having HTT giving an incidence rate of 0.7%. The incidence in preterm infants was 1.6%. Of the 426 infants with mild HTT, 238 (55%) were male, 188 (45%) were female. Trisomy 21 was diagnosed in 15 infants (3.5% of the cohort). The main indications for thyroid function testing were maternal history of thyroid disease (52%, n=221), prolonged jaundice workup (25%, n=106) and follow up of an elevated newborn screen bloodspot TSH level (16%, n=67).

Conclusion: We report an overall HTT incidence rate of 0.7%. HTT was more common in preterm infants and those with Trisomy 21. The incidence of HTT was higher than previously reported in the literature, suggesting that this condition may be underrecognised and underreported.