Characteristics of patients with normal T3 and T4 and a low TSH response to TRH.

The nature of the thyroid disorder presented by patients with normal T4 and T3 but blunted TSH response to TRH has not been clarified. In this study, we compared thyroid function tests in 16 such patients with those of 14 controls and 10 hyperthyroid patients. Basal total T4, free T4, total T3, iodine uptake and cholesterol of the study group were similar to controls but significantly (P less than 0.001) lower than in hyperthyroid patients, except for cholesterol which was higher. In contrast, the basal TSH, increase in TSH after TRH stimulation, and decrease of T4 during T3 suppression tests were similar to data obtained in hyperthyroid patients but significantly (P less than 0.001) lower than in controls. Pulse rate was mid-way between the control and the hyperthyroid group. Thyroid stimulating antibody (TSAb) was measured with human thyroid cells in culture; the assay was positive in four subjects in the 16-patient group and in all hyperthyroid patients tested. TSH stimulation test showed a hyporesponse in iodine uptake in the four patients with positive TSAb (26 +/- 29%), as well as in hyperthyroid patients (6 + 5%). However, there was a hyper-response to TSH (213 +/- 52%) in the remaining 12 patients in the group, none of whom had TSAb. Thus TSAb is not seen as responsible for the thyroid disorder in the majority of patients with normal T3 and T4 and absent or blunted TSH response to TRH; surprisingly, most of these patients have thyroid hypersensitivity to TSH. These two characteristics, absence of TSAb and hypersensitivity to TSH, delineate a thyroid disorder clearly different from Graves' disease.