THE EFFECT OF BUTYL-4-HYDROXY-3:5-DIIODOBENZOATE ON THE AVAILABILITY OF THYROID HORMONES TO PERIPHERAL TISSUES IN ISOTOPICALLY EQUILIBRATED RATS

Abstract

The effect of BHDB is determined on: a) the rate and mode of disappearance of [¹³¹I] l-thyroxine in thyroidectomized, l-thyroxine maintained rats, and b) the concentration of iodine-containing compounds in the body and excreta of thyroidectomized rats, isotopically equilibrated with l-thyroxine or l-triiodothyronine. Labelled thyroxine is rapidly excreted into the intestines and faeces of BHDB-treated rats and this alteration precedes a much less marked decrease of the urinary radioiodide. In isotopically equilibrated rats it is shown that during the first few days of BHDB administration there is a decrease of the urinary iodide, which later reverts to normal or, in the case of triiodothyronine maintained rats, increases well above normal values. The alterations of urinary iodide excretion are more than accounted for by opposite changes in the faecal excretion of iodinated compounds. BHDB results in a depletion of the hormonal pool in the peripheral tissues of thyroxine-maintained rats, but this is not so in the case of triiodothyronine-maintained animals. It is concluded that there is more evidence in favour of the interpretation that BHDB alters the availability of the thyroid hormone to peripheral tissues than in favour of an inhibition of peripheral deiodinating systems. It is suggested that the marked loss of thyroxine from the body of BHDB-treated rats might be the underlying cause of its »antithyroxine« action.