Abstract
Long-term treatment with varying doses of thyroxine caused a marked increase in uptake of muscle, but a single dose caused a significant decrease 24 hours after injection. The uptake by the thymus was markedly decreased, measured both for the whole gland and per unit weight, and occurred independently of the duration of treatment. This decrease was parallel with the involution of the gland. The possibility of using this response in the biological standardization of thyroid extract and thyroxine preparations is discussed. The effects of thyroxin and of cortisone are compared.