Endocrine Abstracts

It has been claimed that the use of the TRH test and the nocturnal TSH surge test might uncover the diagnosis of the so-called 'hidden' central hypothyroidism, in a substantial proportion of euthyroid cranially irradiated children. In our study of 37 euthyroid adult cancer survivors and 34 matched normal controls, patients had significantly (P<0.05) higher basal and stimulated TSH levels and a slightly slower TSH decline between 20 and 60 min during the TRH test; none had blunted or exaggerated responses. 9 patients (27%) had delayed TSH decline (hypothalamic TRH test) but normal free T4. The maximum TSH surge calculated from the average of the highest 3 sequential samples (TSH peak) and the average of lowest 3 sequential samples (TSH nadir) in the whole 24- hour profile period was slightly but significantly reduced in the patients. One normal subject and 2 patients had values lower than the normal of 47%. However, the nocturnal TSH surge was greatly reduced or even negative in 9 normals (26%) and 6 patients (16%), not due to genuine loss of diurnal rhythm, but simply due to a shift in the timing of the peak TSH (acrophase) and/or the nadir TSH to outside the recommended sampling times of 2200-0400 h and 1400-1800 h, respectively; thereby potentially leading to an erroneous diagnosis of 'hidden' central hypothyroidism. The normality of free T4 levels and the wide discrepency between the high rate (30%) of these TSH abnormalities and the very low rate of overt secondary hypothyroidism (3-6%) after prolonged periods of post-irradiation follow-up strongly suggest that, in the vast majority of patients, these abnormalities in TSH dynamics represent subtle functional disturbances in the h-p axis rather than genuine pathology that may progress with time. We suggest that, in this context, the use of the term 'hidden' central hypothyroidism is inappropriate, as these subtle changes may not have any clinical significance.