The Effect of Dietary Protein Restriction on Serum Thyroxine Levels of Pregnant or Growing Swine

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Abstract
Two experiments were conducted with pregnant Yorkshire pigs to study the effects of low protein diets (3% or 0.5% protein) on serum thyroxine levels of gilts, their fetuses and the rehabilitated progeny. In one experiment pregnant pigs were fed an 18%, 3%, or 0.5% protein diet throughout gestation. At 10, 13, and 15 weeks of gestation, fetuses were removed from the uterus after the gilt had been bled to death. In the other experiment, newborn pigs from another set of pregnant pigs fed 18%, 3%, or 0.5% protein during gestation were cross-fostered at 2 days of age to control lactating dams until they were weaned at 4 weeks to a standard diet. There was a decrease in maternal thyroxine level during gestation irrespective of protein intake while there was a significant rise in fetal thyroxine in all groups. However, thyroxine levels were consistently lower in the progeny of dams severely deprived of protein during gestation compared with the other two groups. In a third experiment 3-week old pigs from well-nourished dams were fed a control (18% casein) diet, an energy restricted diet (18% casein) or a low protein diet (7% casein) for 8 weeks followed by 8 weeks of consuming the control diet. During depletion, serum thyroxine was significantly higher in control pigs compared to protein or energy restricted pigs. After 8 weeks repletion there was no difference in thyroxine level of control and energy restricted pigs but the level in protein restricted pigs was significantly lower. It is concluded that protein restriction in prenatal or early postnatal life may result in long-term impairment of thyroid hormone synthesis or release.