ULTRASENSITIVE TSH ASSAY AFTER TRH INJECTION IN THYROID-CANCER PATIENTS UNDER SUPPRESSIVE TREATMENT

The TRH stimulation test was performed in 54 subjects with treated thyroid cancer, who had undetectable basal TSH levels under levothyroxine (group 1). Thirty-two normal subjects were used as controls (group 2). Serum free T4 and free T3 were measured under basal conditions by RIA. Serum TSH was measured by a highly sensitive immunoradiometric method before, and 30 and 60 minutes after a 250-mu-g iv bolus injection of TRH. The sensitivity of the TSH assay was 0.03 mU/l. After the TRH injection in group 1, TSH remained undetectable in 28 cases (group 1 a); in the remaining 26 patients (group 1 b), the mean TSH peak was 0.18 +/- 0.25 mU/l. In group 2, TSH increased from a mean basal value of 1.39 +/- 0.72 mU/l to a mean peak value of 10.93 +/- 4.37 mU/l. The comparison between groups 1 a and 1 b showed that the dosage of levothyroxine and the serum levels of free T4 and free T3 were significantly higher in patients with no TSH response. In conclusion, in treated thyroid cancer with undetectable basal TSH levels, the TSH response to TRH is absent or blunted. The TRH test may be useful to evaluate the degree of the artificial hyperthyroidism. The patients should be given the smallest possible dose of levothyroxine in order to minimize chemical hyperthyroidism.