Thyroidal release after oral administration of TRH in normal subjects.

The oral administration of 40 mg TRH to 17 normal volunteers induced a greater TSH response in females than in males, contrasting with an identical pattern of total T3 and T4 (TT3, TT4) and of free T3 and T4 (FT3, FT4). TSH peaked at 180 min while thyroid hormones (total and free) reached their maximum at 360 min. The drop in the T4 to T3 ratio (total and free hormones) observed at 360 min is consistent with an initial preferential T3 secretion. At 360 min the thyroglobulin (Tg) levels were still in the basal range while at 24 h a significant increase was observed, essentially in female subjects. Thus T3, T4 and Tg seem to be sequentially secreted by the thyroid in response to a prolonged TSH stimulus.