Constant iv infusions of TRH for 4 h were administered to normal men in dosages of 0.5 (n = 5), 2.0 (n = 10) and 5.0 (n = 4) μg/min and to women in a dosage of 0.5 μg/min (n = 4). Control 4-h saline infusions were performed in 4 men and 4 women. Measurements at regular intervals were made for serum levels of TSH, PRL, T4, T3 and 3,3′,5′-triiodo-L-thyronine (reverse triiodothyronine or rT3). Serum levels of TSH increased in 2 phases. The first phase started within 5 min after the infusions began and reached a plateau at approximately 50 min which lasted until 90 min. After 90 min, a second phase of increase in serum TSH levels occurred which continued until 120 to 150 min. The two phases of increase could reflect the existence of two pools of TSH in the pituitary, one requiring longer stimulation for release than the other, as has been suggested for LH. In contrast, serum levels of PRL during the same infusions reached maximal values by 30 to 45 min and then declined gradually in spite of continued TRH stimulation, with no second phase of increase. Serum levels of T3 were significantly increased by 180 min during the infusion of all TRH dosages used. Levels of T4 were significantly increased by 240 min during the 2.0 and 5.0 μg/min infusions. No significant increases were found in rT3. These results are consistent with the interpretation that the major source of circulating rT3 is not the thyroid, but peripheral deiodination of T4.
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Bremner et al. (1977) studied this question.