MODIFICATION OF THERMOGENIC CAPACITY IN NEONATAL PIGS BY CHANGES IN THYROID STATUS DURING LATE-GESTATION

This study was designed to determine the effects of hypothyroidism dur ing late fetal life in pigs on (1) the perinatal pattern of plasma lev els of thyroxine (TT4), total 3,5,3'-triiodothyronine (TT3) and free T -3 (FT3), and liver 5'-deiodinase activity, and (2) the early postnata l development of thermoregulation. Fetal hypothyroidism (test animals) was induced by feeding the sow a high glucosinolate rapeseed diet. Pl asma levels of thyroid hormones, thyroid gland weights and liver 5'-de iodinase activity of control animals increased during late gestation ( P<0.01). The early postnatal period was characterized by a surge in th yroid hormone levels during the first 6 h (P<0.05), followed by a tran sient decrease at 12 h and a second rise by 24 h after birth. This sur ge was much higher (P<0.01) for TT3 than for TT4, but liver 5'-deiodin ase activity did not change during the first 24 h of Life. Fetal hypot hyroidism was characterized by lower plasma levels of thyroid hormones (P<0.05), and lower hepatic 5'-deiodinase activities (P<0.01) than in control fetuses at 110 d of gestation. During the first 6 h of life, test pigs had lower levels of TT4 (P<0.05) but exhibited a greater pos tnatal surge in TT3 and FT3 (P<0.05) than did the controls. The minima l and summit metabolism of the control pigs increased markedly (P<0.01 ) during the first 2 d of life, without any significant change in ther mal body conductance, suggesting that this age-related improvement in thermoregulation was due to the development of the ability to produce heat. The involvement of thyroid hormones in the postnatal increase in heat production is suggested by the following observations: (1) a fet al hypothyroid state impaired the thermogenic response of the newborn pig soon after birth, as shown by a greater drop in rectal temperature (P<0.01) and a reduced summit metabolism (P<0.01); (2) the progressiv e increase in both minimal and summit metabolism observed in control a nimals up to 48 h of life was preceded by an early postnatal surge in thyroid hormones; (3) the faster rise (P<0.05) in thermogenic capacity (summit minus minimal heat production) observed in test than in contr ol pigs between 2 h and 24 h of life was also associated with a greate r early postnatal surge in plasma TT3 (P<0.05). In conclusion, these r esults suggest a major role for thyroid hormones in the control of the rmogenesis in the newborn pig.