Responses of Thyroid Activity to Feed Restriction in the Goat

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Abstract
A study of the effect of restriction of feed intake on the deiodination, fecal excretion, distribution and secretion of thyroid hormones was carried out in a group of 8 young male goats. In the control goats fed ad lib no changes in the concentration of circulating thyroxine were observed with increasing age or body weight during the experimental period; however, an increase in thyroxine secretion rate per animal was apparent with increasing age or body weight of the goats. In feed-restricted goats, no changes in the peripheral deiodination of [125I]thyroxine were observed, though significantly reduced fecal excretion of [125I]thyroxine was seen in goats restricted to 50 or 20% of their ad lib feed intake. Both thyroxine turnover rate and thyroxine distribution volume were reduced by feed restriction. The secretion rate of thyroxine was reduced to 45% of the rate in ad lib fed animals when feed intake was restricted to 20%. Calculation of thyroxine secretion rate per kilogram0.75 body weight showed a highly significant correlation with feed intake. The marked reduction in thyroid activity occurring when feed restriction was imposed would be expected to cause a physiologically significant reduction in metabolic activity.