Abstract
It is well-known that thyroidal 131I-uptake is not suppressed by the T3-suppression test in Graves' disease. However, it has not been elucidated whether serum TSH is suppressed or not. As the basal level of TSH in Graves' disease is generally very low, a comparision of TSH levels before and after the T3-suppression test is difficult. An investigation was made of changes in serum T3, T4, TSH before and after the T3-suppression test in patients treated for Graves' disease (Group I.....non suppressed patients whose TSH decreased after the suppression test, Group II.....suppressed patients, Group III.....non suppressed patients in whom a TSH decrease after the suppression test was not confirmed). After daily administration of 75γ of T3 for 8 days, serum TSH was significantly reduced from 16.5±2.7μU/ml (mean±SE) to 0.6±0.3/μU/ml in Group I, whereas thyroidal 131I-uptake was not suppressed (26.4±3.7% before; 26.6±3.4% after). Immediately after the suppression test serum T4 did not show a significant decrease in non-suppressed patients (Group I, III), but in suppressed patients (Group II) serum T4 was significantly reduced from 8.6±0.3μg/dl to 6.7±0.5μg/dl.
The conclusions were as follows :
1. The non-suppressibility of 131I-uptake may be due to thyroidal autonomy or the existence of thyroid stimulators other than TSH.
2. In general, during recovery from Graves' disease the secretion of pituitary TSH occurs earlier than the suppression of the thyroidal gland.
3. It is suggested, that to evaluate the T3 suppression test, reduction of serum T4 is a more useful indicator than changes in 24-hr thyroidal 131I-uptake.
