Thyroid stimulation test with urinary T3 concentration as an index of thyroid response

Ten USP units of TSH (thytropar) were administered intramuscularly daily for three days to five euthyroid controls, to thirteen patients with chronic thyroiditis [four clinically euthyroid patients with normal serum TSH (Group I), five clinically euthyroid patients with elevated serum TSH (Group II) and four clinically hypothyroid patients with high serum TSH (Group III)], and to five patients with primary hypothyroidism. Effects of TSH on the thyroid 24-hr uptake of 131I, 24-hr urinary T3 excretion and serum T3 concentration were assessed: In euthyroid controls, basal urinary T3 excretion was 0.76 +/- 0.26 (mean +/- S.D.) micrograms/day, increased to 1.31 +/- 0.16 micrograms/day on the first day, to 2.33 +/- 0.75 micrograms/day on the second day, and to a maximum of 2.78 +/- 0.65 micrograms/day on the third day of TSH administration. In Group I, basal urinary T3 excretion was normal (0.58 +/- 0.16 micrograms/day). Following TSH administration, the value increased to 1.60 +/- 0.62 micrograms/day, but the rate of increase was less than that in euthyroid controls. In Group II, basal urinary T3 excretion was normal (0.71 +/- 0.37 micrograms/day), but did not increase following TSH administration. In Group III, basal urinary T3 excretion was significantly low (0.32 +/- 0.07 micrograms/day), and did not increase after TSH administration. In patients with primary hypothyroidism, basal urinary T3 excretion, 0.12 +/- 0.10 micrograms/day, was lower than that of Group III, and did not increase after TSH administration. Positive correlations were found between urinary T3 excretion and serum T3 concentration before and after TSH administration. In contrast to basal serum T3 concentration and urinary T3 excretion, mean basal thyroid uptake of 131I in patients with chronic thyroiditis was higher than that of euthyroid controls. However, a positive correlation was found between the increased rate of urinary T3 excretion and the increase in thyroid 131I uptake after TSH administration. These observations indicate that the changes in urinary T3 excretion are reliable indices of the thyroid response to TSH administration in various states of thyroid function.