SUPPRESSION OF THE TSH RESPONSE TO TRH BY THYROXINE THERAPY IN DIFFERENTIATED THYROID CARCINOMA PATIENTS

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Abstract
Absent response of serum thyrotropin (TSH) after stimulation with 200 .mu.g synthetic thyrotropin-releasing hormone (TRH) was used as a criterion of adequate suppression of TSH secretion in the treatment of thyroid carcinoma patients with thyroxine [T4]. The mean causing total suppression of the response was 223 .mu.g of T4 per day. At this dose level about 40% of the patients had serum T4 concentrations above the upper reference interval and only 10% had elevated triiodothyronine concentrations. In some patients the TSH response to TRH varied between absent and low normal when tested at long intervals. The ideal dose of T4 is obviously slightly higher than the smallest one causing total suppression of the TSH response to TRH, i.e., about 250 .mu.g a day. The individual dose must be found using the TRH stimulation test because serum thyroid hormone levels cannot be used as a guideline for adequate dosage. In some patients the thyroid remnant of apparently normal thyroid tissue was not totally suppressed although the T4 dose was definitely above the level causing suppression of the response to TRH.