The effects of iopodate on the serum iodothyronine pattern in normal subjects

The ingestion by normal subjects of 3 g of sodium iopodate, which is widely used in routine oral cholecystography, resulted in significant decreases of serum total and free T3 to a nadir on day 4 which averaged 43% and 40%, respectively, below initial mean values. Total and free rT3 increased markedly to a peak on day 3, 244% and 189%, respectively, above initial mean values. Total and free T4 and free T4 index rose to a maximum on day 4, but these changes were not statistically significant. A marked TSH increase was also seen, most evident on day 3. All these changes reverted to baseline values by day 14 at a time when serum total iodide was still markedly elevated. It is concluded that the changes observed after iopodate were not due to alterations in serum binding proteins nor to an effect on thyroid gland by the large iodine component of iopodate, but were consistent with an effect on the peripheral metabolism of T4. Difficulty in interpreting routine thyroid function tests may occur for up to 14 days after oral cholecystography with iopodate.