Free thyroid hormone levels during the postnatal period in the pig

Sequential observations on serum concentrations of dialyzable (DFT4,%) and absolute free thyroxine (FT4), free triiodothyronine (FT3) and free reverse-triiodothyronine (FrT3) were conducted in piglets (n = 10) from birth to 21 days of age. Mean FT4 and FT3 were found to be significantly raised during the first 3 days of life, with the highest value (0.520 +/- 0.070% DFT4, 551 +/- 0.1 pmol/l and 0.965 +/- 0.020% DFT3, 18.8 +/- 1.2 pmol/l) between 0 and 12 h after birth. Later, a decrease was observed until the end of the first week followed by a steady and relatively low level during the second and third postnatal weeks. Dialyzable fraction of rT3 increased to a peak value of 48 h of age (0.621 +/- 0.044%) and then decreased til day 7, showing a steady level during the second and third weeks of life. Absolute FrT3 concentration remained low and relatively constant except for a higher value (3.76 +/- 0.44 pmol/l) between 12 and 24 h after birth. The FT4:FT3 and FT4:FrT3 ratios were high between 0 and 6 h of age. The FT3:FrT3 ratio reached the highest value (about 11.0) between the second and fifth days, followed by a fall to 4.7-7.5 during 2-3 weeks after birth.