The effects of tetraiodothyroacetic and triiodothyroacetic acids on thyroid function in euthyroid and hyperthyroid...

The effects of tetraiodothyroacetic (Tetrac) and triiodothyroacetic acids (Triac) on thyroid function have been investigated in euthyroid and hyperthyroid subjects. 50, 100, 200, 400 or 800 micrograms of Triac were administered to 8 euthyroid volunteers three times (tds) over a 24 hour period. 3 X 800 micrograms Triac/24 h was sufficient to cause a significant reduction in serum T3. Tetrac, given as an iv bolus of 3600 microgram, produced a sustained reduction in serum T3 for up to 4 days after the injection. Intermediate doses of Tetrac (1200 micrograms) or Triac (400 micrograms tds) significantly reduced the TSH response to TRH (66% and 43% respectively). Seven hyperthyroid patients received Triac 200 micrograms tds for 2 days, and in 2, a rapid decrease in serum T3 was seen. Similar changes in serum T3 were also produced with iodide administration. The results suggest that 1) in euthyroidism, Tetrac and Triac act directly at the pituitary level to inhibit the TSH response to TRH; 2) in some cases of hyperthyroidism, Triac produces a block in T3 secretion by virtue of the iodide produced by its metabolism.