Rats were made hyperthyroid by repeated sc injections of l-thyroxine (T4) in a two-week period. At several time-points after the last injection, the triiodothyronine (T3) and T4 concentrations in plasma and red and white skeletal muscle were determined by a radioimmunoassay. This was done to investigate the relationship between the change in thyroid-hormone concentration in plasma and muscle and to see whether this change was similar in both muscle types. The results show that:
1. Information about the T3 and T4 concentrations in the muscles of the hyperthyroid rats can only be obtained by direct measurement in the muscles and cannot be gathered from the plasma-T3 concentration at any time-point or from below-normal plasma-T4 concentrations.
2. In hyperthyroid rats T4 and T3 are cleared more rapidly from the plasma than from the skeletal muscle.
3. A proportionally higher T3 concentration is present in the red compared with the white skeletal muscle of hyperthyroid rats.