From the Research
Thyroid hormone levels in term neonates should be interpreted using age-specific reference intervals, with a lower limit of 20.5 pmol/L for free thyroxine (fT4) at day 3-7 and 15.3 pmol/L at day 13-15, as established by the most recent and highest quality study 1. When evaluating thyroid hormone levels in term neonates, it is essential to consider the physiological changes that occur during the first weeks of life.
- At birth, there is a surge in thyroid-stimulating hormone (TSH) that peaks at 30 minutes after delivery, followed by a gradual decline to 5-10 μIU/mL within 48-72 hours.
- This TSH surge triggers a rise in thyroxine (T4) and triiodothyronine (T3) levels, with T4 reaching 10-22 μg/dL and free T4 2-5 ng/dL in the first week of life.
- T3 levels typically range from 100-300 ng/dL. These elevated levels are crucial for neonatal adaptation to extrauterine life, particularly for neurological development, thermogenesis, and metabolic regulation. Clinicians should be aware that premature infants have lower thyroid hormone levels than term neonates, and that maternal thyroid disease, medications, or iodine status can affect neonatal thyroid function. Screening for congenital hypothyroidism is routinely performed between 24-72 hours after birth, with TSH values above 20-40 μIU/mL (depending on laboratory standards) warranting further investigation, as supported by other studies 2, 3. This screening is essential as early detection and treatment of thyroid disorders prevents developmental delays and intellectual disability. In addition to screening, it is crucial to monitor thyroid hormone levels in preterm infants, especially those with lower gestational age, to confirm the diagnosis of congenital hypothyroidism, as suggested by a study on preterm neonates 4. Overall, the use of age-specific reference intervals for thyroid hormone levels in term neonates, as established by the most recent and highest quality study 1, is essential for accurate diagnosis and treatment of thyroid disorders in this population.