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:T3 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.
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Ślebodziński et al. (2009) studied this question.