SERUM TSH RESPONSES TO INTRAVENOUSLY AND ORALLY ADMINISTERED TRH IN MAN AFTER THYROIDECTOMY FOR CARCINOMA OF THE THYROID

Abstract

SUMMARYFourteen patients whose thyroid glands had been ablated for carcinoma were given intravenous or oral thyrotrophin releasing hormone (TRH) at intervals after stopping replacement thyroid hormone. Despite continuing the TRH infusion, serum thyrotrophin (TSH) rose to reach a peak at 120 min after the start of the infusion, and then fell again towards the pre‐treatment values. Significant responses were seen by 4 days after stopping triodothyronine (T3), but progressively increased with longer intervals off T3. Oral TRH gave a similar pattern of responses. Patients maintained on thyroxine (T4) and changed to T3 4 weeks prior to treatment had smaller TSH responses to TRH than patients maintained continuously on T3. It would seem that: (1) TRH promotes TSH release faster than it promotes synthesis of new hormone under the conditions studied, and (2) that long‐term administration of T3 suppresses TSH secretion from the pituitary in response to TRH less than thyroxine. While it is possible to elevate the serum TSH levels of these patients with thyroid carcinoma into the rane normally seen in primary hypothyroidism by the use of TRH prior to administration of radioiodine without production of clinical symptoms of hypothyroidism, we do not yet know whether such elevation is of benefit in treatment.