In the fetus, T4 and TSH levels rise progressively after midgestation; T3 levels increase later after 30 weeks (Figure 20-2).
In full-term infants, the rapid shift in ambient temperature (from 37°C to 23°C) stimulates a striking rise in TSH, which peaks at ~70 μU/mL within 45 to 60 minutes after birth. This is followed by increases in both T4 and T3, which attain levels (T4, 13 to 22 μg/dL; T3, 250 to 400 ng/dL) seen in hyperthyroidism in older children and adults. T4 and T3 decline progressively after the first week of life but remain slightly higher than those in older children throughout the first 6 to 8 months. TSH levels decline after the first week but can be as high as 7.5 μU/mL before 6 months of age.
In preterm infants, the basal T4, T3, and TSH levels are low; this reflects immaturity of the hypothalamic-pituitary-thyroid axis. Differentiation of this hypothyroxinemia of prematurity from central hypothyroidism or the sick euthyroid syndrome can be difficult.
Findings that suggest true central (hypopituitary) hypothyroidism include midfacial hypoplasia, midline cleft lip or palate, nystagmus (seen with optic nerve hypoplasia), midline defects in CNS development such as holoprosencephaly, third ventricular hemorrhage, microphallus, and abnormalities in the levels of other pituitary hormones (eg, low GH, cortisol, ...