The aim of this retrospective study of 43 patients (32 females) aged 26 to 84 years old (mean 62 +/- 14,5) was to assess the value of the TRF test in cases with normal static endocrine levels and isolated arrhythmias: 25 atrial fibrillations, 3 atrial flutters, 10 sinus tachycardias, 2 junctional tachycardias, 2 focal atrial tachycardias and 1 atrial extrasystoles. There was underlying cardiac disease in 24 patients. Thyroid function was assessed by T4, T3 and the TRF test. There was no relationship between cardiac disease and arrhythmias with the levels of TSH 25 minutes after TRF. The reactivity to TRF was low: 5,8 +/- 5,6 microU/ml. Nineteen patients were hyperthyroid (TSH at O and 25 min: 1,7 +/- 0,5 and 2 +/- 1 microU/ml respectively), in the following three eventualities: - Clinical hyperthyroidism and raised hormonal levels, the TRF confirmed the diagnosis in 10 out of 11 cases; - Clinical hyperthyroidism and normal hormone levels: the TRF confirmed the diagnosis in 8 out of 14 cases; - Normal clinical examination and normal hormone levels: the TRF test showed a hyperthyroid reaction in 1 out of 18 cases. T3 and T4 measurements are often misleading. The TRF test with 2 blood samples would appear to be essential, especially in patients over 60 years of age. The hyperthyroid population was older (68,5 +/- 8 years) and the mean values of the thyroid investigations (T3, T4 and STH at 25 minutes) were significant. Underlying cardiac disease was detected in over half these cases. Atrial fibrillation was the commonest arrhythmia. Each case of arrhythmia, whether or not related to hyperthyroidism, justifies echocardiography.