Quantitative Observations With the Thyroxine Suppression Test of Thyroid Function*

The thyroidal I131 uptake rate, one hour after injection of I131, was measured in 450 subjects before and after the administration of ι-thyroxine for about three weeks. In 57 thyrotoxic patients without prior medication, the uptake rate showed a small, but significant mean rise after ingestion of thyroxine; in 51 thyrotoxic patients previously treated with antithyroid drugs there was a small, insignificant mean rise after ingestion of thyroxine. When submitted to the suppression test, most of the 345 euthyroid subjects showed a marked fall in the initially elevated thyroidal I131 uptake rate. There was, however, a small region of overlap between the two types of patients, amounting to about 0.7 per cent of the total euthyroid group and 7.3 per cent of the total hyperthyroid group. The assessment of patients in these overlapping regions is discussed.