THE EFFECT OF THYROXINE ON PERIPHERAL THYROID HORMONE METABOLISM

Abstract

SUMMARY Blood samples obtained from normal people after an oral dose of 0·4 mg. l-thyroxine showed a depression in the uptake of [131I]tri-iodothyronine ([131I]T3) by the red cells, whereas the uptake by a resin sponge was increased. This depression did not occur when the test was carried out at 20° instead of at 37°. It was reversed when methylthiouracil, which is known to inhibit deiodination, was added to the blood in vitro. These findings are indicative of deiodination as the cause of the depression of T3-red cell (RBC) uptake. T3-RBC uptake was similarly depressed after the administration of l-tri-iodothyronine, and it was concluded that deiodination was promoted in the red cell system whenever there was an increase in the level of circulating thyroid hormone. The radioactive product of deiodination, which may be an artifact rather than a natural metabolite, has not been identified. Analysis of plasma obtained from blood after incubation with [131I]T3 under the conditions of the T3-RBC test, has not shown any [131I]iodide other than that present as an impurity in the [131I]T3 preparation. It could be shown that the product of deiodination is bound by the red cells but is eluted from them more readily than [131I]T3. It is suggested that the fall in T3-RBC uptake after thyroxine is due to deiodination which results in the formation of some radioactive product which is eluted from the red cells in the washing procedure.