[Value of the oral TRH test for the evaluation of thyroid function in impaired TSH reserve. Preliminary report]

Oral TRH (40 mg) exerts a marked and protracted stimulatory effect on TSH release. The potency of oral TRH in stimulating TSH and T3 was evaluated in 26 euthyroid patients with absent or impaired TSH response in the i.v. TRH test (200 micrograms). All were given i.v. and oral TRH at an interval of at least 3 days. Based on the response to i.v. TRH, they were divided into "non-responders" (TSH increment less than 1.0 microU/ml) and "low-responders" (TSH increment 1.0--3.7 microU/ml). The results were compared with those in 11 patients with overt hyperthyroidism. 10 "non-responders" were additionally tested with 400 micrograms TRH i.v. to study the effect of a supramaximal dose. Most patients unresponsive to i.v. TRH had a pronounced response to oral TRH (TSH increment 0.1 +/- 0.1 and 4.0 +/- 0.9 microU/ml (mean +/- SEM) respectively, p less than 0.005). The "low-responders" in the i.v. test had a significantly enhanced response to the oral dose (2.4 +/- 0.3 and 7.6 +/- 1.6 respectively, p less than 0.005). The euthyroid patients with treated Graves' disease with a weak response to i.v. TRH showed a distinct response to the oral dose. On the other hand, plasma TSH in patients with overt hyperthyroidism failed to increase either after intravenous or after oral TRH. In contrast to oral TRH, the supramaximal dose of 400 micrograms i.v. TRH did not enhance the TSH-response in the "non-responders". In most patients with absent or impaired response to i.v. TRH, a marked increase in T3 could be demonstrated (0.5 +/- 0.1 ng/ml and 0.6 +/- 0.1 ng/ml in "non-responders" and "low-responders" respectively, p less than 0.01); thus, oral administration allows simultaneous stimulation of TSH and triiodothyronine. Oral TRH can be considered the test of choice for the investigation of patients with diminished TSH reserve.