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Figure 15-2. Postnatal changes in of T4, free T4, TBG, T3, rT3 and TSH according to gestational age. Values determined in babies born at gestational age of 23-27, 28-30, 31-34 and 37 weeks or more are reported. Note the increase in T4, free T4 and TBG in the full term infant in the first week of life. T3 also rises strikingly, while rT3 and TSH decline. The increase in T4 and free T4 is blunted in infants <35 weeks, and not seen at all in very premature infants in whom thyroid hormone levels may actually decline. (Redrawn from Williams et al (10). See text for details).