T4, T3 and reverse T3 in the plasma of rats during the first 3 months of life.

A very sensitive radioimmunoassay for reverse T3 in rat plasma is described. The concentrations of T4, T3 and reverse T3 in plasma of male and female rats were determined during the first 3 months of life. T4 concentrations reached a maximum value of 5.2 +/- 0.7 microgram/100 ml (mean +/- SD) when the rats were about 16 days old. As of day 28 the average serum T4 was approximately 4 microgram/100 ml. Concentrations of T3 reached a maximum value of 168 +/- 34 ng/100 ml (mean +/- SD) during the 16-20 day period. A plateau value of about 40 ng/100 ml was maintained after about 70 days. For reverse T3 a maximum value of 17.1 +/- 4.2 ng/100 ml (mean +/- SD) was found in rats between 13 and 16 days old, followed by a steep decrease to 8.2 +/- 0.9 ng/100 ml around the 28th day and a subsequent continuous slow increase. For the T3/reverse T3 ration (as a parameter of the direction of the conversion pathway) the highest value of 16 was found during the fourth week of life. After 70 days the T3/ reverse T3 ratio was only 3.5. The reciprocal fluctuations between T3 and reverse T3, as found in man and sheep, are less pronounced in the rat. The very high metabolic clearance rate of reverse T3 compared to that of T3 could be the explanation.