Ultrasonic scanning has been used to estimate changes in goitre size during and after treatment with a daily dose of either 100 μg thyroxine (T4) (n = 30), 150 μg T4 (n = 33), 200 μg T4 (n = 17) or 60 μg triiodothyronine (T3) (n = 30) for one year in patients with diffuse non-toxic goitre. The thyroid volume was measured before and every 3 months during and 3 months after withdrawal of therapy. All 4 treatments resulted in a decrease in goitre size after 3 months' therapy (P < 0.05) but only in the patients given 200 μg T4 and 60 μg T3 was the decrease long lasting during additional 9 months' therapy. The mean reduction in thyroid volume after 12 months therapy was 30 ± 26 sd and 22 ± 30 (sd)%, respectively. The free-T4-index was found significantly raised throughout the treatment with all three T4 doses, whereas the free-T3-index did not change. During treatment with 60 μg T3 no persistent rise in free-T3-index was seen while free-T4-index was decreased all 12 months.
No increase in goitre size was observed 3 months after withdrawal of the T3 therapy (n = 15), whereas all goitres in the T4 treated groups had returned to the pre-treatment values at that time. No rebound phenomenon was observed. The serum T3 and T4 levels were normalized 3 months after withdrawal in all four groups. Seven out of 8 patients on 200 μg T4 daily and 20 out of 25 patients on 60 μg T3 daily had no TSH response to TRH (200 μg iv) after 3 months' therapy. Forty-three out of 61 patients in the 4 groups showed no TSH response to TRH after 3 months' therapy. Only the group of patients showing no TSH response to TRH after 3 months' treatment had a significant decrease in goitre size in contrast to the group with a positive response to TRH.