PARADOXICAL MODULATION OF THYROTROPE RESPONSIVENESS TO TRH DURING TREATMENT OF THYROTOXIC PATIENTS - APPARENT ABSENCE OF FEEDBACK REGULATION

Eight patients with active thyrotoxicosis were followed up to 1 yr after the onset of antithyroid treatment. At different time intervals during the investigation period, TRH [thyrotropin releasing hormone] tests were performed and total T4 [thyroxine] and T3 [triiodothyronine] basal levels were measured. The TRH-induced TSH release was delayed in all patients in spite of a normalization of the circulating levels of thyroid hormones. The delay varied according to the patients and lasted, in some cases, for as long as 24 wk after the normalization of the thyroid hormone levels. Administration of thyroid hormones, together with methimazole, did not seem to prevent pituitary thyrotrope responsiveness; moreover, it did not provoke an inhibition of the TRH-induced TSH release once thyrotrope reactivity was fully restored. The feedback mechanism does not seem to be the main modulator of the pituitary responsiveness to TRH in these circumstances.