A methodological study in measuring T3 and T4 concentration in red and white skeletal muscle and plasma of euthyroid...

T3 and T4 concentrations were determined in plasma and red and white skeletal muscle of the rat. Because of the small tissue samples (+/- 300 mg), the ultra-sensitive Wick radioimmunoassay (RIA) for serum was adapted for determination in ethanol extracts. The dilution curves of the plasma and tissue extracts showed excellent parallelism with the standard curves for both T3 and T4. The mean T4 level found in female rats (n = 6) was 22.6 +/- 5.2 ng/ml in plasma and did not differ significantly between red (1.85 +/- 0.28 ng/g) and white (1.90 +/- 0.25 ng/g) skeletal muscle. The mean T3 level in 11 normal female rats was 0.629 +/- 0.0098 ng/ml in the plasma and was significantly higher in the red muscle (2.07 +/- 0.26 ng/g) than in the white muscle (1.65 +/- 0.20 ng/g). The higher T3 levels found in the red muscle as compared with the white muscle may help to elucidate the different responsiveness of these muscle types observed in altered thyroid states.