Reduced thyroid function after thyrotropin stimulation.

The course of serum T4 and T3 return to baseline after TSH stimulation was studied in two groups at six normal subjects over 28 days after im bovine TSH (b TSH; 0.15 U/kg). In the first group of six subjects, serum bTSH rose from undetectable levels to a mean peak of 5.6 +/- 0.5 ng/ml (mean +/- SE) at 2 h, and fell below detectable levels by 24 h with a t1/2 of 7 +/- 1 h. T4 rose to a peak 59 +/- 10% above basal levels within 24 h, returned to basal levels on day 7, then dropped below basal levels on days 9-24, with a nadir of -16 +/- 4% on day 14. Free T4 paralleled T4 levels. T3 rose to a peak 104 +/- 28% above basal at 24 h, then fell faster than T4, reaching basal levels by day 4. During the period of low T4, T3 was at or below basal levels. Human TSH (h TSH) concentration dropped when T4 and T3 rose, but did not rise above basal levels when T4 and T3 fell below basal levels. Neither a T3 elevation nor an increased percentage of free T4 was present during the time of reduced T4 levels. The same pattern of thyroidal response was seen in the second group of six subjects. In this second group, hTSH response to repeated TRH challenge was studied. During the period of reduced T4 and T3, hTSH response to TRH was diminished. On day 28, T4, T3, hTSH, and hTSH response to TRH returned to basal levels. We conclude that the brief elevation of T4 and T3 after bTSH stimulation exerts a suppressive effect on the pituitary which extends beyond the period of elevated thyroid hormone levels, and that delay in pituitary recovery is the mechanism of the decreased thyroid function after acute bTSH stimulation.