[Standardization of a new thyroid gland suppression test by means of 3000 microgram L-thyroxine]

The suppressibility of the TSH-thyroid axis one week after a single dose of 3000 microgram L-thyroxine was studied in 21 euthyroid subjects. In all cases the normal thyroid function was proven by euthyroid values for thyroxine (T4), free thyroxine index (FT4-I), triiodothyronine (T3), radioiodine uptake, PBI-131, a normal scintiscan and by a physiologic TSH response to thyrotropin-releasing hormone (TRH). Compared to the baseline values the mean suppression of radioiodine uptake at 2,24 and 48 h was 61.8% (+/- 32.9), 62.0% (+/- 15.3) and 62.1% (+/- 13.4) respectively. Because of the large variation at 2 h this value can be disregarded. If the 24 h value shows a clear suppression over 50%, no further measurement is necessary. Mean serum levels of thyroxine and free T4-index rose from 8.2 +/- 1.0 to 13.0 +/- 2.1 microgram/100 ml and from 7.7 +/- 1.9 to 15.1 +/- 3.4 respectively on the third day, to the upper limits for normal controls. Triiodothyronine increased from 1.69 +/- 0.42 to 2.08 +/- 0.38 ng/ml and remained definitely below the level of 2.60 ng/ml observed in overt thyrotoxicosis. In single subjects T4, FT4-I or T3 achieved slightly hyperthyroid levels. In view of the effectiveness of a single dose of 3000 microgram T4 with good suppression of radioiodine uptake and TSH production after stimulation with TRH without noticeable side effects, the classical T3 suppression test should be abandoned and replaced by this test, which latter can be used for the same diagnostic and therapeutic purposes.