Levels of TSH, T4, and T3 change following birth (Fig. 15.5). A sharp rise in serum TSH, commencing with delivery, peaks approximately thirty minutes after birth. This decreases rapidly after four hours, then more slowly over the next forty-eight hours, to reach normal levels within a week of birth. Persistently high TSH values (greater than 50mu/l) occur five days after birth in congenital hypothyroidism (see Fig. 15.21). T4 levels rise rapidly during the first forty-eight hours after birth, with values reaching the hyperthyroid range. Levels peak between twenty-four and forty-eight hours and fall back into the normal adult range within two weeks. T3 levels are low at delivery and rise rapidly due to increased peripheral conversion of T4 to T3. Peak values occur approximately twenty-four hours after birth and then fall to normal levels over subsequent days.
