The combined T3/TRH test in depressed patients and healthy controls

It is well established that depressed patients show a blunted TSH response in the TRH-stimulation test. However, it has not been investigated so far whether pre-treatment with 3,5,3′-triiodothyronine (T3) is able to further suppress the TRH-induced TSH response in depressed patients or whether it may cause an escape-phenomenon with paradoxically enhanced TSH stimulation in a subsequent TRH test. In 20 drug-free depressed patients (eight men, 12 women) suffering from a major depressive episode according to DSM-IV criteria and in 20 age- and sex-matched healthy controls, the single TRH-stimulation test (administration of 200 μg TRH at 09:00 h) was carried out followed by a combined T3/TRH test (pre-treatment with 40 μg T3 at 23:00 h the night before; administration of 200 μg TRH at 09:00 h the next day). Compared to the controls, the depressed patients showed a significantly blunted TSH response in the single TRH test. However, the percentage suppression of TRH-induced TSH stimulation after pre-treatment with 40 μg T3 was comparable in the depressive patients (61.07%) and the healthy volunteers (64.20%). Prolactin secretion did not differ between patients and controls either in the single TRH test or in the combined T3/TRH test. Apparently, in contrast to the hypothalamo-pituitary-adrenocortical (HPA) system, no disturbance of feedback control in regulation of the hypothalamo-pituitary-thyroid (HPT) axis secretion can be demonstrated in depressed patients when using the combined T3/TRH test.