[Fruste form of hyperthyroidism manifested by auricular arrhythmia. Importance of the assay of the free fraction of thyroxine (FT4) and the role of the TRH test].

The TRH test was used to detect hyperthyroidism in 87 patients aged from 38 to 85 years who presented with atrial arrhythmia with or without heart disease. The patients had no clinical evidence of thyrotoxicosis, and total thyroxine (T4), free thyroxine index (FTI) and triiodothyronine (T3) values were normal. Hyperthyroidism was diagnosed in the 18 patients (21%) with negative TRH test; 15 of them had high free thyroxine (FT4) levels. The most common causes of hyperthyroidism were "warm" nodules in 7 and iodine overload in 10. Adding an anti-thyroid treatment to the hitherto unsuccessful anti-arrhythmic treatment resulted in a return to sustained sinus rhythm in 50% of cases. FT4 levels became normal in all. This study indicates that all patients with atrial arrhythmia, with or without heart disease, should be investigated for occult hyperthyroidism. It also demonstrates the value of FT4 assays to detect the disease. The TRH test is only required as a second-line exploratory method in some patients, notably those with iodine overload.