Effect of triiodothyronine or iodide on the thyroidal secretion in vitro: Inhibition of TSH- and dibutyryl-cyclic-AMP induced endocytosis.

T3 or iodide, at the concentration of 10- minus 3M, inhibits TSH- or dibutyryl-cyclic AMP-induced thyroidal intracellular colloid droplet formation, while leaving the TSH-induced increase in cyclic AMP levels intact. The effect of T3 appeared not to be mediated through the iodide derived from deiodination of T3. Those results clearly demonstrate that T3 or iodide acts at the point beyond cyclic AMP generation and prior tothe formation of intracellular colloid droplets. This is also the site at which microtubule active reagents exert their influence. Whether T3 or iodide fits this category or acts in as yet undetermined manner remains to be established.