[Biological detection of hormonal overdose in treated peripheral thyroid insufficiency. Comparison of ultrasensitive TSH and the TRH tests. Therapeutic consequences].

Fifty-two patients with peripheral hypothyroidism receiving hormone replacement therapy at conventional dosages and having normal free thyroid hormone levels underwent a TRH test (200 micrograms). This test was found areactive (delta TSH less than 4.0 microU/ml) indicating therapeutic overdosage in 38 cases (73 p. 100). It was possible to reestablish reactivity to the TRH test in 20 of these patients by progressively reducing the dosage to an average of 70 p. 100 of the initial dose. This reduction had no effects on body weight, heart rate or serum lipids. These results show that the plasma concentrations of thyroid hormones are poor indicators of overdose and underline the limitations of the concept of group normality to which that of individual normality is to be preferred. The treatment of hypothyroidism is much easier to adapt using the TSH test. Since the introduction of ultrasensitive methods of assaying this hormone the value of the TRH test has been questioned. We have defined the limits beyond which the TRH test is of no value and the basal TSH dosage adequate. There remains a zone (basal TSH between 0.15 and 1.00 microU/ml in our laboratory) within which the result of the TRH test cannot automatically be deduced from that of the basal TSH.