Effect of a single oral dose of L-thyroxine (150 micrograms) on serum thyroid hormone and TSH concentrations in clinically euthyroid goitrous patients.

Serum TT4, FT4, TT3, FT3 and TSH, concentrations were measured before and 0.5, 1, 2, 3, 4, 5 hours after a single oral dose of L-T4 (150 micrograms) in 21 clinically euthyroid goitrous patients on a long-term therapy program. The mean basal TT4 concentration, 8.85 +/- 0.52 micrograms/dl (mean +/- se), increased significantly at the second hour (10.04 +/- 0.52 micrograms/dl, p < 0.001) and reached the highest value at the third hour (10.35 +/- 0.55 microgram/dl, p < 0.001). Subsequently the mean serum TT4 concentration remained elevated until the fifth hour (10.20 + 0.60 microgram/dl, p < 0.001). The mean basal value of FT4 concentration, 8.66 +/- 0.40 pg/ml, increased significantly at 30 minutes (9.19 +/- 0.40 pg/ml, p < 0.01) and reached the highest value at the third hour (11.81 +/- 0.56 pg/ml, p < 0.001). The mean basal serum FT3 concentration, 3.56 +/- 0.10 pg/ml, increased significantly 30 minutes (3.72 +/- 0.11 pg/ml p < 0.05) after L-T4 administration; the mean serum FT3 concentration was significantly higher than the basal level and 1 and 2 hours as well, and reached the highest value at the third hour (3.86 +/- 0.09 pg/ml). The mean serum TT3 concentration did not show any significant change at any time. The mean basal TSH value (2.73 +/- 0.42 microUI/ml) declined progressively. At 30 minutes it decreased significantly (2.29 +/- 0.38 microUI/ml, p < 0.01) and reached the lowest value at the fifth hour (1.39 +/- 0.26 microUI/ml, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)