TRIAC (3,5,3'-TRIIODOTHYROACETIC ACID) HAS PARALLEL EFFECTS AT THE PITUITARY AND PERIPHERAL TISSUE-LEVELS IN THYROID-CANCER PATIENTS TREATED WITH L-THYROXINE

OBJECTIVE To investigate whether the addition of 3,5,3'-triiodothyroacetic acid (TRIAC) to thyroxine (T4) treatment can suppress TSH secretion without inducing thyrotoxicosis at the periphery. DESIGN Thyroid cancer patients were studied with different treatment modalities: T4 at supraphysiologic dose (2.5 +/- 0.3-mu-g/kg/day) and after reduction to a physiologic dose (1.8 +/- 0.3-mu-g/kg/day); then with the addition of TRIAC 500 or 1000-mu-g/day to the physiologic T4 treatment dose. PATIENTS Twenty-two patients who had total thyroid ablation for differentiated thyroid carcinoma. MEASUREMENTS Clinical and biological parameters of thyroid hormone action studied included heart rate, serum creatine phosphokinase, testosterone-oestradiol binding globulin, procollagen III and osteocalcin levels. RESULTS The addition of TRIAC induced a significant and dose-dependent decrease in serum TSH levels and parallel effects on peripheral tissues. Compared to the suppressive T4 treatment dose, the addition of TRIAC to the physiologic T4 dose resulted in greater inhibition of TSH secretion in only 50% of the patients. The effects at the periphery of both treatment modalities were similar for a comparable level of TSH suppression. CONCLUSIONS Even at low dose and when combined with T4, TRIAC has parallel effects on the pituitary and peripheral tissues. There is no justification for the use of TRIAC as suppressive treatment in thyroid cancer patients.