Prematurity is the Main Factor for Transient Congenital Hypothyroidism in Greece, a Recently Iodine-Replete Country

Background/Objectives: Neonatal screening programmes for thyroid function testing, based on Thyroid Stimulating Hormone (TSH) assessment, detect not only Permanent Congenital Hypothyroidism (PCH), but also Transient Congenital Hypothyroidism (TCH). In Greece, newborn screening for congenital hypothyroidism (CH), using TSH measurement in dried blood spots (Guthrie card), began in 1979 through the Institute of Child Health (ICH). Although the general Greek population is considered iodine-replete, most pregnant Greek women are mildly iodine-deficient according to WHO criteria. The aim of this retrospective study was to record the cases of TCH and the main causative factor over a 10-year period (2010–2019) in Greece, when the country was iodine-replete. Methods: The number of births in Greece between 2010 and 2019 were retrieved from the Hellenic Statistical Authority (ELSTAT) archives reaching 952,109 births, while the total number of newborns assessed through the ICH was 951,342 (99%). During this period, 22,391 newborns were detected with TSH >7 mIU/L after the second check on the initial Guthrie card. Among those, 17,992 underwent re-testing with a serum sample. Out of the re-tested newborns, 1,979 were screened positive for Congenital Hypothyroidism (CH) and immediately began treatment with levothyroxine. We followed up with families, paediatricians, and paediatric endocrinologists to determine whether L-thyroxine therapy had been successfully discontinued for at least two months after the child’s third birthday. Successful contact was achieved with 889 individuals. From this group, 329 children had successfully discontinued thyroxine, classified as TCH. The remaining 560 cases were considered to have PCH. Demographic data, including gender, gestational age, and birth weight, were collected from the archives of the ICH. Maternal data, including thyroid medication use and the presence of elevated thyroid autoantibodies during pregnancy and childbirth, were also recorded. Newborns were categorized as full-term or premature. To identify TCH cases not influenced by maternal factors during gestation, such as medication use or elevated thyroid autoantibodies, we have excluded these cases, defining the Target Group of our study. Results: Logistic regression analysis revealed that, while controlling for all other predictor variables, the odds ratio of transient hypothyroidism was 2.078 (95% CI: 1.530 to 2.821, p = 0.001) for prematurely born children compared to those born at term. The effect of other factors on TCH versus PCH was not significant. ANOVA indicated an interaction between prematurity or permanent hypothyroidism and birthweight centile (F = 7.861 & 5.205, p = 0.005 & 0.023, respectively). Conclusions: Prematurity is the main factor for Transient Congenital Hypothyroidism in a recently iodine-replete country, such as Greece.