Relation of triiodothyronine and reverse triiodothyronine administration in rats to hepatic L-triiodothyronine aminotransferase activity

The effect of T3 and reverse T3 administration on muscle protein catabolism during fasting as measured by 3-methylhistidine excretion

Since recent studies have indicated that measurement in urine of the amino acid, 3-methylhistidine, accurately reflects the extent of muscle catabolism, and because it has been suggested that thyroid hormones may influence muscle breakdown, especially during fasting, the effect of T3 and reverse T3 (rT3) administration on the excretion of 3-methylhistidine was examined in obese subjects during fasting. The mean (+/- SE) 3-methylhistidine excretion in patients fed an egg protein diet (devoid of meat protein) was 256 +/- 35 mumoles/day and decreased to 190 +/- 14 mumoles/day during fasting. T3 administration (100 microgram/day x 5 days) increased 3-methylhistidine excretion to 304 +/- 37 mumoles/day during its ingestion and to 485 +/- 46 mumoles/day in the T3 posttreatment interval. T3 doses of 10 microgram every 4 hr (q4h) for the first 6 days of fasting also appeared capable of increasing 3-mehis excretion whereas 5 microgram T3 q4h administered during the first 6 days of fasting did not increase 3-mehis excretion. Reverse T3 administration (80 microgram q6h) during fasting was associated with a mean 3-methylhistidine of 130 +/- 13 mumoles/day, a value no higher than in patients fasted alone. These observations suggest that: (1) skeletal muscle catabolism decreases during fasting: and (2) pathophysiologic doses of T3 (60 microgram/day or more), but not reverse T3, enhance muscle catabolism during fasting.

Recent developments in thyroid hormone metabolism: interpretation and significance of measurements of reverse T3, 3,3'T2, and thyroglobulin

The development of specific radioimmunoassays has allowed measurements of 3,3',5'-triiodothyronine (reverse T3), 3,3'-diiodothyronine (3,3'T2), and thyroglobulin to be performed in serum and in various biological fluids both in normal and in altered states of thyroidal economy. The physiology, kinetics, and metabolic actions of reverse T3, 3,3'T2 and thyroglobulin are reviewed. Presently, it appears that reverse T3 and 3,3'T2 measurements in amniotic fluid and cord serum may potentially be useful in diagnosing fetal or neonatal thyroid dysfunction, and serum thyroglobulin measurements appear to be important as a measure of the efficacy of treatment of patients with differentiated thyroid carcinoma.