Sequential Observation of Changes in Thyroxine, Triiodothyronine and Reverse Triiodothyronine during the Postnatal Adaptation of the Pig

Principal serum iodothyronines, T₄, T₃ and rT₃, have been simultaneously measured by a specific radioimmunoassay in piglets kept for sequential observation with the mother from 1 to 21 days of age. During several hours postnatally, a high concentration of the hormone was noted as a result of an enhanced secretory activity of the thyroid gland. Later, a progressive decrease with a nadir at about day 3 was observed, followed by a second rise in the hormone level. Changes in T₃ and rT₃ levels from birth to the end of the 1st week, paralleled those for T₄. The T₃:rT₃ ratio of about 2 during the 1st week fell to about 1.5 during the 3rd week as a result of the progressive increase in rT₃ and decrease in T₃ concentrations. The T₄:rT₃ ratio fell with the progressing age to the level observed in post-weaned pigs. At birth, the newborn pig seems to possess a low capability of 5-deiodinase enzyme system thus the converting enzymes first work towards T₄–T₃ conversion while T₄–rT₃-converting ability progressively increases with age. The role of this variability for the neonatal thermogenic stability is discussed.