Thyroid hormone levels related to energy and nitrogen balance during weight loss and regain in adult sheep

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Abstract
: Caloric deprivation (feeding 50 and 25% of the original maintenance requirement for 126 days), followed by overnutrition (feeding 133 and 200% of the original maintenance requirement) for 37 days, were induced in 7 adult sheep. Before and after the periods of undernutrition and after overnutrition the animals were fasted (total energy withdrawal) for 5 days. Energy- and nitrogen-balances were determined and related to concentrations of thyroxine (total: T4; free: FT4), 3,5,3′-triiodothyronine (total: T3; free: FT3) and 3,3′,5′-triiodothyronine (reverse T3,rT3). Caloric deprivation led to decreased T3 and FT3 levels, overnutrition to increased T3 and FT3 concentrations. T3 was significantly correlated with energy- and nitrogen-balances (r = 0.73 and 0.71, respectively; P< 0.001). T4 and FT4 behave similar to T3, but correlations between T4 levels and energy- and nitrogen-balances were low. In contrast to T3, rT3 levels increased during energy withdrawal and decreased during overnutrition. After an overnutrition for months, an additional fasting for 5 days did not increase rT3 levels, however. rT3 was significantly correlated with energy- and nitrogen-balances (r = −0.50 and −0.50, respectively; P < 0.01).