Responses of thyroid activity to feed restriction in the goat

A study of the effect of restriction of feed intake on the deiodination, faecal excretion, distribution and secretion of thyroid hormones was carried out in a group of eight young male goats. In the control goats fed ad libitum 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 faecal excretion of [125I]thyroxine was seen in goats restricted to 50 or 20% of their ad libitum 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 libitum fed animals when feed intake was restricted to 20%. Calculation of thyroxine secretion rate per kilogram 0-75 body weight showed a highly significant correlation with feed intake. It was concluded that the marked reduction in thyroid activity occurring when feed restriction was imposed would be expected to cause a physiologically significant reduction in metabolic activity.