Decreased Serum 3,5,3′-Triiodothyronine (T3) and Abnormal Serum Binding of T3in Calorie-Deficient Rats: Adaptation after Chronic Underfeeding*

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Abstract
Both starvation and feeding of a low protein diet have dramatic effects on serum thyroid hormone concentrations and on the serum binding proteins for thyroid hormones in rats. We examined whether similar changes might be seen in another model of undernutrition, namely underfeeding without alteration of dietary composition, and in particular whether such changes would disappear after prolonged alteration in diet (adaptation). Male rats aged 21 days were put on five different levels of intake of a diet of normal composition (18% protein, 70% carbohydrate), and animals from each dietary group (n = 8-10) were killed after 30, 60 or 100 days of underfeeding. After 30 or 60 days of underfeeding, significant direct correlations were observed between growth rate (used as an index of the degree of underfeeding) and serum T3 (RIA), percent free T3 (equilibrium dialysis), and serum free T3 (T3 .times. percent free T3). When underfeeding was prolonged to 100 days, however, there was no correlation between growth rate and present free T3, while the correlations between growth rate and serum free T3 was weak (r = 0.33). Qualitatively similar changes were seen when animals given five different levels of food intake were killed at three body weight milestones rather than three separate age milestones. Polyacrylamide gel electrophoresis of serum thyroid-binding proteins revealed that the low percent free T3 in underfed rats seen after 60 days of underfeeding was associated with the development of a thyroid-binding globulin not normally found, but this had disappeared by 100 days of underfeeding. We conclude that nutrition-related changes in serum thyroid hormone variables show adaptation over time. Because of changes in serum binding of thyroid hormones caused by undernutrition, total serum thyroid hormone concentrations may not be an accurate reflection of thyroid status in any investigational study in which an experimental treatment leads to decreased food intake.