Simultaneous observations of iodothyronine content in the thyroid gland, serum and thyroxine 5'- and 5-monodeiodinase activity in liver, during the neonatal period of the pig.

Serum T4 and rT3 were high at about 4-12 h after birth, then they decreased to a nadir on day 3 (rT3) and day 7 (T4). Serum T3 concentration fell immediately after birth but then increased to a relatively stable level during the next 2-6 weeks, then fell after weaning. Reciprocal concentration profiles of T4, T3 and rT3 in the thyroid were found. The thyroidal iodothyronine content increased significantly after weaning. In the liver, 5'-monodeiodinating activity, low after birth, rose until day 3 and then decreased concomitantly with T3 in serum. The 5-monodeiodinating activity, high at birth, fell to a nadir at about 3 weeks. No changes in 5- and 5'-deiodinase activity after 3 weeks were observed. Opposite to the variations in absolute content, the iodothyronine relative proportion in thyroid tissue was practically unchanged until weaning time (6 weeks), when they rose. Serum T3/T4 and rT3/T4 ratios increased with age until weaning. The post-weaned pigs had T3/T4 and rT3/T4 ratios about two times smaller than 6-weeks-old pigs. Serum rT3/T3, high after birth, decreased with age. Summarizing, the results indicate that neither changes in the thyroid iodothyronine content nor in the liver T4-monodeiodinating activity can solely account for variations in serum TH during the early neonatal period in the pig. It is suggested that the rapid variations in serum TH levels can reflect changes in the thyroidal secretory activity in preferential T3 secretion and/or blood disappearance rates.