Effects of a single high oral dose of thyroxine on the pituitary-thyroid-axis

Ten euthyroid and 9 hypothyroid volunteers were orally administered 2 mg L-thyroxine (T4) to study the interaction between substitutive and suppressive effects of a single high T4 dose. After a significant rise the serum T4 concentration for 5 days in euthyroid and for 11 days in hypothyroid patients an inhibition of basal and of TRH stimulated TSH release was observed. Maximal inhibition of the TSH response in hypothyroid patients occurred 1 to 7 days after the individual T4 peak. This interval was significantly correlated to the extent of the respective T4 rise. In euthyroid subjects TSH response was significantly inhibited for 8, in hypothyroid patients for 22 days. In all but 3 of the euthyroid patients there was a significant inhibition of the thyroidal 132I uptake on day 8. Normalization of thyroidal 132I uptake and of pituitary TSH secretion generally coincided.