What is the approximate half-life of serum T4 in newborns?

• A. 1-2 days
• B. 3-4 days
• C. 5-6 days
• D. 7-8 days

Answer: (B)

The concept being tested is how long it takes for serum thyroxine (T4) levels to drop by half in a newborn when production and clearance are in the neonatal range. In newborns, the circulating T4 half-life is shorter than in adults, commonly cited as about 3 to 4 days. This reflects the rapid but immature turnover of thyroid hormone in early life, plus the transition from maternal T4 to the infant’s own production after birth. Adults typically have a longer half-life for T4, around 7 days, so the neonatal value sits somewhere between rapid clearance and mature metabolism.

Therefore, 3–4 days is the best estimate for the neonatal half-life. A shorter interval like 1–2 days would be unusually fast for T4 turnover, while longer intervals such as 5–6 or 7–8 days align more with adult physiology and are not typical for newborns. This timing matters clinically because it influences how quickly serum T4 levels respond to changes in production or therapy in the newborn period.