Further Studies on the Inhibitory Effect of Diazo Dyes on Thyroid Function in the Rat

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Abstract
In an attempt to determine the mode of action of diazo dyes in suppressing thyroid function, several of the possible pathways were investigated in the rat. Five days after the injections of trypan red and trypan blue, thyroid weight, thyroidal radioiodine uptake and thyroidal radioiodine release had decreased markedly. However, the inhibitory effect of trypan red was much less than that of trypan blue. By injecting graded doses of ACTH, significant increases in adrenal weight were produced, similar to those caused by diazo dye treatment, but no significant changes were found in thyroid weight, thyroidal radioiodine uptake or thyroid hormone synthesis, indicating that the hyperfunctioning pituitary-adrenal system has no causative role in hypofunction of the thyroid. It was found that trypan blue blocked the synthesis of thyroxine but not of DIT, an effect very similar to that produced by hypophysectomy. The uptake of I131 T4 by red blood cells was augmented significantly by both trypan red and trypan blue, but to a lesser extent by Evans blue. It is suggested that diazo dyes may compete with thyroxine for binding sites on serum proteins, and free thyroxine thus produced may inhibit TSH secretion by the pituitary.