High Incidence of Hypothyroidism Within a Month in Neonates with Down Syndrome Who Are Euthyroid at Birth: Results of A Prospective Study

The objective was to test for hypothyroidism (HYT) in infants with Down Syndrome (DS) who tested euthyroid on state newborn screening for congenital HYT. In 39 infants with DS who were euthyroid at birth, serum TSH, total T4 (TT4) and free T4 (FT4) levels were measured during 15 to 120 days of age. TSH, TT4 and FT4 were measured by standard techniques. Depending up on age specific TSH and thyroid hormone (TH) levels, babies were classified as: Euthyroid (normal TSH and normal TH), Primary HYT (high TSH and low TH), compensated HYT (high TSH and normal TH) and sick thyroid (low TSH and low TH). Comparisons among different groups were performed using two-sample t-tests. Twenty-nine babies (72%) remained euthyroid (TSH 6.1 ± 2.6 mIU/ml, TT4 10.8 ± 1.8 µg/ dl and FT4 1.57 ± 0.4 ng/dl). Six (18%) babies developed primary HYT (TSH 23.5 ± 10, TT4 5.1 ± 1.2, FT4 1.17 ± 0.2) within a month of life needing T4 therapy. While 4 (10%) babies developed compensated HYT (TSH 29.3 ± 10, TT4 10.6 ± 3.6 and FT4 1.83 ± 0.3), one (2.5%) had sick thyroid syndrome (TSH 4.6, TT4 4.6 and FT4 1.1). All four babies with compensated HYT were euthyroid by four months of age. Despite normal newborn screening at birth, the incidence of any HYT was much higher (28 %) than the normal neonatal population (1/2000-4000). More importantly, HYT developed within a month of age. Recommendations by the AAP about retesting TH function in infants with DS should be revised.