Triac (3,5,3'-triiodothyroacetic acid) partially inhibits the thyrotropin response to synthetic thyrotropin-releasing hormone in normal and thyroidectomized hypothyroid patients.

The effects of a daily oral dose (1.4 mg) of 3,5,3'-Triiodothyroacetic acid (Triac) on thyroid hormone levels (T4, T3 and rT3) and on the TSH and PRL responses to TRH were studied in 15 normal subjects and 5 hypothyroid patients. There were no significant changes in weight, heart rate, reflex time, or serum concentration of either cholesterol or triglycerides after 6 weeks of Triac administration. However, T4 was significantly reduced to a lower mean level (mean +/- SEM, 7.3 +/- 0.7 to 4.3 +/- 0.6 microgram/dl) in the control group. T3 and rT3 concentrations increased, possibly due to a cross-reaction with Triac in their respective RIAs. The peak TSH response to TRH in the normal subjects was 17.6 +/- 3.4 muU/ml and fell significantly to 2.0 +/- 0.8 muU/ml after Triac administration. In the hypothyroid subjects the mean serum TSH level was significantly reduced from 136 +/- 66 to 12.6, 10.5, and 11.6 muU/ml in the weeks after Triac administration. The mean peak response of both TSH and PRL after TRH (206 muU and 44.8 ng/ml, respectively) declined significantly to 63.4 muU/ml and 24 ng/ml. It was concluded that this dose of Triac partially inhibits the synthesis and secretion of TSH and PRL without any major peripheral metabolic effects.