Different rates of thyrotropin suppression after total body scan in patients with thyroid cancer: effect of regular doses of thyroxine and triiodothyronine.

Abstract

Listen

Translate article

The relative rates of T4 and T3 in inducing TSH suppression when therapy is re-started after Total Body Scan (TBS), in patients with differentiated thyroid cancer, were evaluated in two groups of 9 patients, each. The doses adopted were those usually considered to be TSH suppressive in chronic administration (T4 = 3.4 ± 0.3 µg/day/kg bw; T3 = 1.1 ± 0.04 µg/day/kg bw). At time 0, TSH levels were elevated in all patients, ranging from 58 to 147 µU/ml; T3 and T4 were below the limit of detectability. After the beginning of the therapy decrease of TSH levels and inhibition of TSH response to TRH was very gradual. It was slower in patients on T4: at day 14 basal TSH level was 23.6 ± 26.7 µU/ml (mean ± SD) and 30 min after 200 µg TRH iv mean TSH level was 99 ± 40 µU/ml; at day 30 mean basal TSH was 1.5 ± 1.2 µU/ml and, after TRH, 5.8 ± 6.9 µU/ml, than in patients on T3: at day 14 mean basal TSH level was 1.8 ± 1.0 µU/ml and after TRH 10 ± 13.9 µU/ml; at day 30 mean basal TSH was 1.0 ± 0.5 µU/ml and after TRH was 2.7 ± 1.4 µU/ml. In patients taking T3, serum levels of T3 rose to normal limits very quickly (at day 3 mean serum T3 level was 159 ± 30 ng/100 ml) and afterwards remained slightly above normal range (at day 7 it was 213 ± 41 ng/100 ml). In patients taking T4, the rise of serum levels of both T3 and T4 was much slower and gradual: at day 3 mean T3 level was 53 ± 15 ng/100 ml; at day 7 it was 99 ± 25 ng/100 ml; at day 14 it was 144 ± 26 ng/100ml; at day 3 mean T4 levels was 2.0 ± 0.6 µg/100ml; at day 7 it was 6.6 ± 1.5 µg/100 ml; at day 14 it was 9.9 ± 1.9 µg/100 ml. These data show that T4 in the usual suppressive dose is not suitable to achieve TSH suppression as quickly as would be necessary in patients with differentiated thyroid cancer when therapy is re-started after TBS. Treatment with T3, 1.1 µg/day/kg bw, induces TSH suppression more quickly. Therefore, it seems advisable in such patients to start suppressive treatment with T3 and to shift to T4 after a month, in order to avoid any unnecessary stimulation by TSH of eventual residual tumor.