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The rate of thyroxine utilization has been measured from the rate of degradation of [¹²⁵I]thyroxine and the concentration of plasma proteinbound iodine in ewe lambs. Repeated measurements were made between 10 and 60 weeks of age in two groups of lambs, one in conditions including temporarily inadequate nutrition and outdoor ambient temperatures, and the other in a controlled environment with free access to food.

The lambs in the controlled environment showed an increase in thyroxine utilization with increasing age throughout the experiment, whereas the 'outdoor lambs' showed a decrease in hormone utilization until after 34 weeks of age. The 'outdoor lambs' also lost bodyweight between the 20th and 26th week of age due to the poor quality of the pasture, and this undernutrition is considered to have caused the lowered rate of thyroxine utilization. After 26 weeks of age the nutrition of these lambs was adequate but thyroxine utilization did not increase until after 34 weeks of age, when the mean environmental temperature was about 5°. It was concluded that undernutrition caused a marked depression in thyroxine utilization of young lambs.