Sir.—The article by Hahn et al1 drew my attention to the significantly greater thyroxine (T4) and triiodothyronine values in 2- to 3-week-old infants.
I have measured the serum T4 level on the third day of life in 1,759 normal, full-term infants with the use of the iodine 125 (Gamma Goat) T4 radiommunoassay kit. The mean ± SD serum T4 levels of the 680 formula-fed infants (16.32±0.09 μg/dL) and the 1,079 breast-fed infants (16.46±0.08 μg/dL) were similar.2 A male breast-fed infant was noted to have a serum T4 value of 2.4 μg/dL, and results of further tests confirmed the diagnosis of hypothyroxinemia.
I also have measured the serum T4 levels in 143 premature infants (33 ±3.1 weeks old) who have recovered from their acute neonatal problems and were being fed with breast milk (n = 39) or formula (n = 104) for one to six weeks
BANAGALE RC. Serum Thyroxine Values in Breast-fed Infants. Am J Dis Child. 1984;138(4):410. doi:10.1001/archpedi.1984.02140420076023