Changes in serum TSH and T4 after T3 suppression tests in patients treated for hyperthyroidism due to Graves' disease.

A study of changes in serum TSH, T4 and T3 before and after the T3 suppression test was done in patients treated for hyperthyroidism due to Graves' disease. In 18 uptake suppressed patients of 49 clinically euthyroid subjects serum TSH was significantly reduced after the T3 suppression test. In 25 of 31 uptake non-suppressed patients a comparison of serum TSH levels before and after the T3 suppression test was difficult to conduct, as the basal level of serum TSH was very low. In 6 patients with high basal serum TSH (greater than 10 μU/ml), it was significantly (P < 0.001) reduced from 13.8 ± 1.4 (mean ± SE) to 0.9 ± 0.3 μU/ml in response to an increase of serum T3 after the T3 suppression test, whereas thyroid 131I-uptake was not suppressed (from 28.0 ± 4.3 to 28.1 ± 3.9%). Thyroid inhibition of TSH secretion was normal in these patients and therefore non-suppressibility of 131I-uptake is not due to abnormal TSH secretion. Concerning the response of TSH to TRH, one-half of non-suppressed patients showed ...