The effect of some iodine-containing radiocontrast agents on iodothyronine secretion from the perfused canine thyroid

The effects of the iodine-containing radiocontrast agents ipodate, iopanoic acid, and iothalamate on the secretion of T4, T3, and rT3 were studied using perfused canine thyroid lobes. The experiments were performed as paired comparisons, with one thyroid lobe acting as a control. At 10(-5) M, the two agents used for cholecystography (ipodate and iopanoic acid) inhibited the TSH-stimulated secretion of T3 [ipodate, 69 +/- 8% of control (P less than 0.05); mean +/- SE; n =4] and rT2 [iopanoic acid 59 +/- 9% (P less than 0.01); iopanoic acid, 61 +/- 9% (P less than 0.05)], whereas T4 secretion was not significantly altered. Iothalamate (10(-3)M), an agent predominantly excreted through the kidneys, did not alter T4, T3, or rT3 secretion. This pattern is compatible with an inhibitory effect on intrathyroidal T4 deiodination to T3 and rT3 by ipodate and iopanoic acid, similar to the inhibition of T3 deiodinases in peripheral tissues by these compounds. At a concentration of 10(-3) M, ipodate induced a hitherto unrecognized pattern of thyroid hormone secretion: a very rapid, sustained and reversible inhibition of the secretion of all three iodothyronines. The mechanism behind this response remains to be elucidated.