Exogenous and Endogenous Suppression of Thyroid-Stimulating Hormone Induces Similar Effects on Thyroidal lodothyronines

Summary

Thyroidal concentrations of T₄and T₃and the T₄/T₃ratio were analyzed in the nodular and paranodular tissues from two groups of patients with suppressed TSH secretion. The first group consisted of 17 patients with nontoxic nodular goitre (NG), 8 of whom received long-term levothyroxine therapy to suppress TSH, while remaining 9 were untreated. The second group consisted of 10 patients with autonomously functioning thyroid adenoma (AFTA), in whom TSH secretion was suppressed due to the adenoma-induced increase in thyroid hormone concentrations. In nodular tissues of NG patients, thyroidal T₄and the T₄/T₃ratio were significantly higher in treated than in untreated patients (0.34 ± 0.05 vs. 0.15 ± 0.02 mol T₄/mol of thyroglobulin (Tg) and 10.9 ± 1.2 vs. 5.2 ± 0.7 respectively). Analysis of paranodular tissues of NG patients also revealed a higher T₄/T₃ratio in treated patients (16.0 ± 2.1 vs. 6.9 ± 0.9), although thyroidal T₃and T₄concentrations in treated and untreated patients were similar. In AFTA patients, both T₃and T₄concentrations were higher in the adenoma than in paranodular tissues (0.14 ± 0.04 vs. 0.02 ± 0.005 mol T₃/mol Tg and. 1.08 ± 0.32 vs. 0.26 ± 0.06 mol T₄/mol Tg), whereas the T₄/T₃ratio was significantly higher in paranodular tissues (23.2 ± 5.9 vs. 9.3 ± 1.8). These results indicate that suppression of TSH induced either exogenously or endogenously results in an increase in the thyroidal T₄/T₃ratio that reflects an increase in T₄and/or a decrease in T₃concentrations. These findungs also support the notion that TSH preferentially stimulates thyroidal T₃production.

Key words

TSH suppression - Thyroidal iodothyronines - Non-toxic goitre - Autonomously functioning thyroid adenoma