Thyroid hormone is essential for development of the brain in early life. Thyroid dysfunction is more common in the first 2-4 postnatal weeks of life in premature infants than in term infants. This study aimed to identify the prevalence and clinical course of thyroid dysfunction in prematurity.

Premature infants admitted to and given neonatal screenings at Dankook University Hospital between April 1999 and March 2008 were included in this study. We retrospectively reviewed medical records and categorized subjects into six groups: normal, hypothyroidism, hyperthyrotropinemia, hypothyroxinemia, delayed onset of hypothyroidism, and delayed onset of hyperthyrotropinemia.

Among 599 subjects, 136 (23%) had initially abnormal thyroid function test (TFT); transient hypothyroxinemia was the most frequent condition (118, 20%). In addition, 8 (17%) of 46 subjects with initially normal TFT levels showed delayed onset of hyperthyrotropinemia with or without low free thyroxine (fT₄). Thyroxine was prescribed for 10 patients (1.7%) due to low fT₄ levels but was discontinued in 9 patients during follow-up. Thyroid scan confirmed ectopic thyroid in one patient.

Thyroid dysfunction was frequently seen in premature infants, but most of the conditions were transient. In addition, some infants showed delayed TSH elevation on routine follow-up. Therefore, a recheck of the thyroid function of premature infants at 3-4 weeks is recommended, even if normal thyroid function is initially seen, especially in prematurity of less than 33 weeks of gestational age or birth weight of less than 2,500 grams.